Chemical Physics Letters (v.442, #1-3)

Contents (vii-xiv).

Theoretical study of reaction mechanism for NCO + HCNO by Weichao Zhang; Benni Du; Changjun Feng (1-6).
This figure shows the potential energy profile along the minimum energy pathways for the reaction of NCO + HNCO. Firstly, IM7 can be formed without an entrance barrier. Then the HCN + OCNO can be formed via TS7. Only a barrier of 14.90 kcal/mol needs to overcome for this process and the energy of TS7 is even 9.82 kcal/mol lower than that of the reactants. Consequently, further direct dissociation of OCNO to form the product CO + NO through the direct C–N bond cleavage via TS5(4).A detailed quantum chemical study is performed on the mechanism of the NCO + HCNO reaction, which has never been studied by theory. The potential energy surface for this reaction is characterized at the B3LYP/6-311++G(d,p) level of theory, combined with high-level CBS-QB3 single point energy calculation. Four possible product channels have been investigated. From the calculations it can be seen that the formation of HCN + NO + CO is the dominant product channel, while the pathway to products of HCNN + CO2 is expected to be minor one, and these conclusions are in good agreement with the experimental results.

IR spectroscopy of 2-fluoropyridine–water clusters in the electronic excited states by Yoshinori Nibu; Ryosuke Marui; Hiroko Shimada (7-11).
IR spectra of S1 states of 2-fluoropyridine–(water) n (n  = 1–3) clusters are observed in a supersonic free jet.The IR spectra of hydrogen-bonded clusters of 2-fluoropyridine (2-FP)–water were studied experimentally in a supersonic free jet. These IR spectra of the clusters were observed with the fluorescence detected infrared depletion (FDIR) technique in the OH stretching vibrational region for the electronic ground and excited states. The frequency decrease of the hydrogen-bonded OH stretching vibration of 2-FP–(H2O)1 with electronic excitation is considerably smaller than that observed for phenol–(H2O)1. All the hydrogen-bonded OH stretching vibrations in 2-FP–(H2O) n become lower in the electronic excited state compared with those in the ground state.

Negative ion photoelectron spectroscopy of copper hydrides by Ryan M.D. Calvi; Django H. Andrews; W. Carl Lineberger (12-16).
Photoelectron spectra of CuH and CuH 2 - provide thermochemical data, and definitively show the existence of both CuH2 X2B2 and HCuH A2Σg.We report photoelectron imaging spectra of CuH and CuH 2 - and their deuterated analogs. The CuH photoelectron spectrum exhibits transitions to the ground (X1Σ+) and lowest excited triplet (a3Σ+) and singlet (A1Σ+) states, with EA(CuH) 0.444(6) eV. The photoelectron spectrum of linear X 1 Σ g + HCuH is dominated by a transition to the linear excited A 2 Σ g + state with a 2.853 eV binding energy. It is accompanied by an extended unresolved vibrational progression involving the bent CuH2 X2B2 ground state. Ab initio calculations show a 0.255 eV energy difference between these two states allow determination of EA(CuH2), 2.60(5) eV.

The time-dependent-wave-packet method has been applied to calculate the time-resolved photoelectron spectra of Br2 molecule. The calculated results are in good agreement with experimental ones. Some phenomena have been revealed.The time-dependent-wave-packet method has been applied to calculate the photoelectron spectra of Br2 molecule with zero time delay. Meanwhile, Br+ ion that mainly resulted from the dissociation of Br2 (C1 Π u) can be seen in the photoelectron spectra of Br2 molecule. We presume that more Br atoms will be ionized later, as the dissociation is nearly complete. In addition, it is found that Br 2 + ( X 2 Σ g + ) has the largest increasing rate as the increase of the laser intensities.

Identification of two conformers of 5-indanol in a supersonic free jet by Sudip Banerjee; Shamik Chakraborty; Partha Pratim Parui; Debnarayan Nath; Tapas Chakraborty; Mihir Chowdhury (21-27).
Discrimination of two conformers of 5-indanol with the aid of both experiment and theory.Two conformational isomers of 5-indanol have been identified in a supersonic jet expansion and their S0  ↔ S1 vibronic spectra have been investigated using laser induced fluorescence excitation, dispersed fluorescence and UV–UV hole-burning spectroscopy. Two conformational isomers appear due to the orientation of the –OH group at the 5-position of the benzene ring. The puckering C(1)H2 frequency has been assigned with the aid of theoretical calculations. The experimental observations have been interpreted by the predicts of ab initio electronic structure calculations performed at the HF/6-31+G and MP2/6-31G level of theories.

Guided ion beams study of ion–molecule reactions at low collision energies: The Li+–acetone adduct formation in the 0.10–1.00 eV center of mass energy range by M. Sabidó; J.M. Lucas; J. de Andrés; J. Sogas; M. Albertí; A. Aguilar; D. Bassi; D. Ascenzi; P. Franceschi; P. Tosi; F. Pirani (28-34).
Under single collision conditions and using radio frequency guides, the reaction between ground state lithium ions and acetone molecules has been studied and the [Li–acetone]+ adduct observed at different relative collision energies.A new apparatus optimised for the study of low-energy ion–neutral molecule collisions, designed and built in the University of Barcelona, is described in this Letter, as well as its use for the study, under single collision conditions, of the acetone–lithium ion system. The cross-section energy dependence in the 0.10–1.00 eV center of mass energy range has been measured in arbitrary units for the Li+–acetone adduct formation. Also, an empirical potential energy surface for the lithium–acetone adduct has been constructed and used to provide a qualitative interpretation of the reaction.

Quantum-chemical calculations on a novel reaction mechanism of CNN with NO by Hui-Lung Chen; Han-Jung Li; Jia-Jen Ho (35-41).
DFT calculations of the possible interaction between CNN and NO was performed at the CCSD(T)/aug-cc-PVTZ//B3LYP/6-31++G(d,p) level. Products P3 (NCO + N2) and P4 (N2O + CN) were the two leading channels with slight energy barriers as compared to the reactants.Quantum-chemical calculations of the possible interaction between CNN and NO have been performed at the CCSD(T)/aug-cc-PVTZ//B3LYP/6-31++G(d,p) level of theory. Fourteen conformers corresponding to local minima of potential energy and sixteen transition structures are located, and various possible reaction paths are probed. Products P3 – NCO + N2 – and P4 – N2O + CN – were the two leading channels with slight energy barriers as compared to the reactants CNN + NO. Employing the Fukui functions and HSAB theory, we are able to rationalize the calculated outcomes.

B3LYP/6-31+G(d) and MP2/6-31+G(d) predict different structures for one Tyr–Gly conformer due to missing dispersion in B3LYP and large BSSE in MP2.B3LYP/6-31+G(d) and MP2/6-31+G(d) calculations predict markedly different structures for one Tyr–Gly conformer. Calculation of the energy profile for rotation around the glycine Cα–N bond reveals one minimum in the B3LYP profile (ϕ gly  = 180°) and two in the MP2 profile (∼75° and 280°). Large intramolecular BSSE values are responsible for masking the 180°-minimum in the MP2 profile: approximate elimination of BSSE in the MP2 calculations – by (1) correction using BSSE values from complexes of phenol and N-formylglycine, (2) the application of local MP2, or (3) employing large basis sets (aug-cc-pVTZ/QZ) and density fitting – yields an unambiguous triple-well potential.

Motion of the encapsulated metal in M@C74 (M = Be, Mg, Ca, Sr, and Ba) is studied from a view of vibronic coupling with the group-theoretical consideration.Motion of the encapsulated metal in M@C74 (M = Be, Mg, Ca, Sr, and Ba) is studied in terms of vibronic coupling using group theory approach. The selection rule for the symmetry of metal motion is presented. This agrees well with experimental findings for Ca@C74. The metal motion can be regarded as a pseudo-Jahn–Teller effect. The electronic structures and adiabatic potential surfaces of M@C74 are calculated using the tight-binding approximation.

Experimental arrangement of two-photon wavepacket interferometry. The femtosecond pulses are injected into a Michelson interferometer. One arm is varied in delay motor with 0.1 μm resolution, and the other arm is controlled by a piezoelectric transducer (PZT), and a monochromator is used to select about 1 cm−1 bandpass for the frequency locked. L1 and L2 is the lens, PMT is a photomultiplier tube, server circuit is for phase-locked loop (PLL).Time-resolved two-photon fluorescence spectra have been investigated based on wavepacket interferometry (WPI), and the wavepacket dynamics of the excited states for 4-dicyanomethylene-2-methyl-6-p-dimethyl-aminostryryl-4H-pyran (DCM) is determined by phase-locked femtosecond pulse pairs. A relative phase between the femtosecond pulse pairs can be maintained as the delay line scanning, and so the two-photon fluorescence signals will be observed to appear periodically recurring features. It indicates the constructive or destructive interference between two-photon wavepackets on the molecular excited states. The experimental results show that the phase-locked WPI has the potential applications in the wavepacket dynamics of the complicated molecular systems.

We propose a scheme for generation and control of entanglement and arbitrary superposition states between molecular vibrational and rotational modes by using three sequential chirped pulses. We demonstrate numerical examples and discuss the effect of molecular characteristics.In this Letter, we propose a scheme for generation and control of the entanglement and arbitrary superposition states between molecular vibrational and rotational modes. We consider molecules in parahydrogen to be excited by linearly polarized chirped pulses in infrared and microwave frequency regions. From the calculations, it turns out that efficient control can be achieved. As a demonstration, we focus on HF and LiH molecules and clarify the effect of molecular characteristics on the state preparation using the molecular vibrational and rotational modes.

New structures of dense nitrogen: Pathways to the polymeric phase by Razvan Caracas; Russell J. Hemley (65-70).
A new high-pressure stable structure of nitrogen is theoretically found, showing new transition pathways between molecular and atomic structures.The existence of new polyatomic structures of nitrogen at high density has been examined based on recent observations for analog systems using first-principles density-functional theory. A structure with Immm symmetry and two molecules per unit cell is found to be stable relative to the known high density diatomic and polymeric phases of nitrogen. The structure is dynamically stable from ambient pressure to at least 90 GPa and thus may represent a new class of observable polynitrogen phases at high pressures and temperatures. Other newly predicted structures may be observed experimentally as stable or metastable phases in different pressure–temperature ranges.

Isomers of dicyanobenzene (DCB) including 1,2-DCB, 1,3-DCB and 1,4-DCB are examined as solids and in the liquid state by Terahertz spectroscopy and compared to Density Functional Theory.Terahertz absorption spectra of three isomeric structures of dicyanobenzene in chloroform solution and solid phase at 298 K are reported. These spectra exhibit enhanced absorption in low THz range compared to most organic systems because of strong coupling to phonon modes. Molecular vibrational spectral calculations show strong correlation with the experiment especially for solution spectra. All intramolecular modes were assigned and intermolecular modes identified. Out-of-plane intramolecular modes at low frequency exhibit ∼75 cm−1 blue shifts as the secondary CN group moves from the para to ortho position on the benzene ring, whereas almost no frequency shift occurs for low-frequency in-plane modes.

Primary processes in photophysics and photochemistry of PtBr 6 2 - complex studied by femtosecond pump–probe spectroscopy by Ivan P. Pozdnyakov; Evgeni M. Glebov; Victor F. Plyusnin; Nikolai V. Tkachenko; Helge Lemmetyinen (78-83).
Scheme of photophysical and photochemical processes for PtBr 6 2 - complex in aqueous solutions with their characteristic times is presented.Ultrafast pump–probe spectroscopy (λ pump  = 420 nm) was applied to study the photoaquation of PtBr 6 2 - complex. The excitation to d–d excited 1T1g state was followed by the formation of an intermediate absorption decaying with characteristic times of 370 fs, 2.2 and 15.2 ps. Two shorter times were attributed to the formation of the lower excited state (3T1g) and its vibrational cooling accompanied by solvent relaxation. The longest time was interpreted as 3T1g transition to the hot ground state (1A1g) and the dissociation product (PtBr5(H2O) complex). The scheme is consistent with the independence of the photoaquation quantum yield on the excitation wavelength.

Optical properties of the new potential infrared-detectors Cu(I)2SO3  · M(II)SO3  · 2H2O (M = Cu, Fe, Mn, and Cd) series and the influence of M(II) exchange by Luciana A. Silva; Marcos A. de Araújo; Jailson B. de Andrade; Kelly A. Silva; Denis G.F. David; Antônio Ferreira da Silva; Iuri Pepe (84-88).
The optical absorption of mixed-valence sulfites were studied and the influence of the M(II) cation. Band gap energies were determined.The optical absorption of mixed-valence sulfites such as Cu(I)2SO3  · M(II)SO3  · 2H2O [M = Cu, Fe, Mn, or Cd] were studied and the influence of the M(II) cation on this parameter. A transmission spectroscopy technique was used to measure the optical band gap energies (E g) in the infrared region and the E g of Cu(I)2SO3  · Cu(II)SO3  · 2H2O was estimated using a numerical fitting method that can be described empirically by E g ( χ P ) = - 0.62 χ P 2 + 1.88 χ P - 0.5 , where χ P is Pauling’s electronegativity of metal M. The obtained values were in the interval 0.85–0.95 eV, representing a narrow E g, which can identify these compounds as potential infrared-detectors.

CdSe quantum dots were grafted onto the planar fluorine-doped tin oxide (FTO) glass (a) and nanostructured SnO2 electrode (b). Optical absorption, photosensitization, and ultrafast carrier dynamics were studied and found to depend strongly on the morphology of the substrates.CdSe quantum dots (QDs) are adsorbed onto nanostructured SnO2 and planar FTO electrodes by a chemical adsorption technique. Optical absorption and the incident-photon-to-current conversion efficiency (IPCE) are studied by using photoacoustic and photoelectrochemical methods. The growth rate of the CdSe QDs depends on the morphology of the substrate. Larger IPCE peak values and larger integrated area for the IPCE spectra are obtained for the nanostructured SnO2 electrodes. The ultrafast carrier dynamics of the CdSe QDs are studied using a transient grating technique. It is found that the electron relaxation of the CdSe QDs depends strongly on the morphology of the substrate.

Fabricating chiral polydiacetylene film by monolayer compression and circularly polarized ultra-violet light by Gang Zou; Hideki Kohn; Yuki Ohshima; Takaaki Manaka; Mitsumasa Iwamoto (97-100).
TDA LB films could be polymerized to form chirality under higher compression ((a) 20 mN m−1), while not under lower compression ((b) 20 mN m−1).We study polydiacetylene films that are pertinent to the problems of mirror symmetry breaking induced by the effects of compression and circularly polarized ultra-violet (UV) light. The subphase is only pure water. After polymerization, polymerized 10,12-tricosadiynoic acid (PTDA) LB films that deposited at the surface pressure of 20 mN m−1 showed obviously chiral properties, however, no obvious Cotton effect was obtained for PTDA LB films that deposited at the surface pressure of 10 mN m−1. In addition, TDA LB films could be polymerized to a designed chirality by using chiral circular polarized ultra-violet light (CPUL).

Ag@SnO2 core–shell structure nanocomposites by Suraj Kumar Tripathy; Hyun-Woo Kwon; Young-Min Leem; Byoung-Gyu Kim; Yeon-Tae Yu (101-104).
The synthesis of Ag@SnO2 nanocomposite particles with core–shell morphology has been reported. The formation of metal oxide shell on the Ag nanoparticles was monitored by the UV–Visible spectroscopy and was confirmed by the TEM images of the resultant core–shell particles. TEM pictures of nanocomposite particles were synthesized by adding 0.5 mL of 40 mM sodium hexahydroxostannate .The synthesis of Ag@SnO2 nanocomposite particles with core–shell morphology has been reported. Ag colloidal particles were synthesized by a well-documented aqueous reduction technique using NaBH4 as reductant, trisodium citrate as a stabilizer and AgClO4 as the source for the Ag4+ ion. The formation of metal oxide shell on the Ag nanoparticles was monitored by the UV–Visible spectroscopy and was confirmed by the TEM images of the resultant core–shell particles. The presence of a SnO2 shell on the silver nanoparticles was also corroborated by XPS results. Optical property of the nanocomposite was investigated by photoluminescence spectroscopy.

Pd, Rh, Ir and Pt adsorption on gold: A theoretical study of different surfaces by H.J. Gotsis; I. Rivalta; E. Sicilia; N. Russo (105-109).
The interaction of atomic Pd, Rh, Ir and Pt with the Au(1 1 1) and Au(1 0 0) surfaces is studied by ab-initio density functional calculations within the generalized gradient approximation (GGA). For the two surfaces, we have determined the preferred adsorption sites, the adsorption structures, spin-projected density of states and local magnetic moments, including relaxation effects. This study allows some common features to be seen and some significant differences that can be explained by an analysis of the electronic structures of both surfaces.The interaction of atomic Pd, Rh, Ir and Pt with the Au(1 1 1) and Au(1 0 0) surfaces is studied by ab-initio density functional calculations within the generalized gradient approximation (GGA). For the two surfaces, we have determined the preferred adsorption sites, the adsorption structures, spin-projected density of states and local magnetic moments, including relaxation effects. This study allows some common features to be seen, in particular in the adsorption energies and geometries for both surfaces and some significant differences such as the considerable broadening of the impurity atom bandwidth on the Au(1 0 0) surface. The origin of these differences is explained by an analysis of the electronic structures of both surfaces.

An NMR and molecular dynamics investigation of the avian prion hexarepeat conformational features in solution by Adriana Pietropaolo; Luca Raiola; Luca Muccioli; Giustiniano Tiberio; Claudio Zannoni; Roberto Fattorusso; Carla Isernia; Diego La Mendola; Giuseppe Pappalardo; Enrico Rizzarelli (110-118).
The effect of proline cistrans isomerisation on the conformational behaviour of avian prion hexarepeat Ac-PHNPGY-NH2 is investigated at acid, neutral and basic pH [a, b, c], revealing a shift from turn-like structures to unordered ones.The prion protein is a copper binding glycoprotein that in mammals can misfold into a pathogenic isoform leading to prion diseases, as opposed, surprisingly, to avians. The avian prion N-terminal tandem repeat is richer in prolines than the mammal one, and understanding their effect on conformation is of great biological importance. Here we succeeded in investigating the conformations of a single avian hexarepeat by means of NMR and molecular dynamics techniques. We found a high flexibility and a strong conformational dependence on pH: local turns are present at acidic and neutral pH, while unordered regions dominate at basic conditions.

Theoretical Karplus relationships for vicinal coupling constants around χ1 in Valine by Reynier Suardíaz; Carlos Pérez; José M. García de la Vega; Jesús San Fabián; Rubén H. Contreras (119-123).
Karplus coefficients for χ1-related vicinal coupling constants in Valine, using DFT calculations are reported and analyzed within NBO methodology.Karplus coefficients for χ1-related vicinal coupling constants in Valine dipeptide model, obtained from DFT calculations are reported. The computed coefficients agree with available experimental data, excepting C1 coefficient that is obtained with negative sign. This apparently anomalous coefficient sign is analyzed within NBO methodology. From this analysis a relationship between the hyperconjugation of the bonding orbitals σ(C–C) and σ(C–H) of the coupling pathway with the vicinal coupling constants is obtained.

Electron detachment of the hydrogen-bonded amino acid side-chain–guanine complexes by Jing Wang; Jiande Gu; Jerzy Leszczynski (124-127).
The vertical electron detachment of the hydrogen-bonded amino acid side-chain–guanine complexes is affected by positive/negative charges.The photoelectron spectra of the hydrogen-bonded amino acid side-chain–guanine complexes has been studied at the partial third order (P3) self-energy approximation of the electron propagator theory. The correlation between the vertical electron detachment energy and the charge distributions on the guanine moiety reveals that the vertical electron detachment energy (VDE) increases as the positive charge distribution on the guanine increases. The low VDE values determined for the negatively charged complexes of the guanine–side-chain-group of Asp/Glu suggest that the influence of the H-bonded anionic groups on the VDE of guanine could be more important than that of the anionic backbone structure. The even lower vertical electron detachment energy for guanine is thus can be expected in the H-bonded protein–DNA systems.

A charged ring model for classical OH(aq) simulations by Ivan S. Ufimtsev; Andrey G. Kalinichev; Todd J. Martinez; R. James Kirkpatrick (128-133).
We propose a new ‘charged ring’ (CR) model for hydroxide anion in aqueous solution.We propose a new ‘charged ring’ (CR) model to better represent the delocalized electron density of the oxygen atom of hydrated OH in classical molecular simulations. This approach corrects a major defect of traditional point-charge models, which inaccurately predict that OH accepts predominantly 5–6 hydrogen bonds in aqueous solution. Classical molecular dynamics simulations with the CR model of OH result in ∼4 accepted H-bonds, in agreement with recent neutron diffraction data and ab initio molecular dynamics results. The observed competition between 3- and 4-coordinate OH(aq) structures is also in good agreement with ab initio results.

Two-electron Breit-Pauli relativistic correction incorporated with one-electron IOFW HamiltonianThe applicability of the lowest-order two-electron Breit–Pauli (2e-BP) relativistic correction incorporated with the infinite-order Foldy–Wouthuysen (IOFW) transformation was examined in heavy and super-heavy elements. Comparing with the non-relativistic Coulomb repulsion term, in cases of moderately heavy noble-gas atoms (He to Ar), the 2e-BP correction gave the SCF energies practically equivalent to those obtained with the Dirac–Fock/Coulomb method. The spin-orbit splittings were also corrected reasonably. On the other hand, in heavy noble-gas atoms (Kr to Rn), the 2e-BP correction tended to overshoot energy lowering, and in a super-heavy atom (E118), evidences of collapse of the 2e-BP correction were observed.

2D-dynamic representation of DNA sequences by Dorota Bielińska-Wa¸ż; Timothy Clark; Piotr Wa¸ż; Wiesław Nowak; Ashesh Nandy (140-144).
A new 2D-graphical representation of DNA sequences is presented. Moments of inertia of the graphs are proposed as descriptors.A new ‘dynamic’ 2D-graphical representation of DNA sequences is presented. The model is based on 2D-plots that have been used before and are easy to visualize, but it removes many degeneracies present in the previous approaches. The moments of inertia of the ‘dynamic’ graphs are proposed as a new kind of descriptor for DNA sequences.

Mid-infrared continuous wave cavity ring down spectroscopy of molecular ions using an optical parametric oscillator by H. Verbraak; A.K.Y. Ngai; S.T. Persijn; F.J.M. Harren; H. Linnartz (145-149).
Continuous wave cavity ring down spectroscopy is used to record rovbrational spectra of molecular ions in direct absorption through supersonically expanding planar plasma.A sensitive infrared detection scheme is presented in which continuous wave cavity ring down spectroscopy is used to record rovibrational spectra of molecular ions in direct absorption through supersonically expanding planar plasma. A cw optical parametric oscillator is used as a light source and combines a broad spectral coverage (2100–3600 cm−1) and narrow bandwidth (7 kHz) with a high output power (∼1 W) and excellent TEM00 mode characteristics. The performance of the technique is demonstrated by recording a rotationally cold spectrum of formyl cation, HCO+, in the CH stretching region. For optimal conditions, absolute ion densities in the plasma expansion amount to 2 × 1010  HCO+-ions/cm3 in the detection zone.

In this Letter, we study the effects of higher order excitations in the relativistic CC calculations for atoms and ions with one valence electron using CCSD, CCSD(T) and its unitary variants. The present study demonstrates that CCSD(T) estimates the ionization potentials (IPs) and the valence electron removal energies quite accurately for alkali atoms and singly ionized alkaline earth ions, but yields unphysical energy levels for atoms and/or ions with partially filled sub-shell like C II, Al I, etc. We further demonstrate that the higher order excitation (as well as electron correlation) effects can be incorporated more effectively through the unitary coupled cluster theory (UCC) compared to the CCSD(T) method.In this Letter, we study the effects of higher body excitations in the relativistic CC calculations for atoms and ions with one valence electron using Fock-space CCSD, CCSD(T) and its unitary variants. The present study demonstrates that CCSD(T) estimates the ionization potentials (IPs) and the valence electron removal energies quite accurately for alkali atoms and singly ionized alkaline earth ions, but yields unphysical energy levels for atoms and/or ions with partially filled sub-shell like C II. We further demonstrate that the higher body excitation effects can be incorporated more effectively through the unitary coupled cluster theory (UCC) compared to the CCSD(T) method.

Covalent bond indices and ionicities from similarity measures by Diego R. Alcoba; Roberto C. Bochicchio; Luis Lain; Alicia Torre (157-163).
The total ionicity of a bonding in the Bray–Curtis metric.This work proposes the use of the mathematical concepts of similarity and distance in metric spaces to relate the covalency and ionicity indices of chemical bondings. Numerical results arising from the Bray–Curtis and divergence measures are reported and discussed in selected molecules. The procedure opens new possibilities to define and to relate complementary quantities.

Comparison of CPU time between the existing diagrammatic scheme and the new matrix-element scheme presented in this Letter.An efficient computational scheme for using the relativistic and non-relativistic quasidegenerate perturbation theory with general multiconfigurational reference functions is implemented. The scheme is based on the matrix element between the reference and the zero- to two-electron ionized determinants. Comparison with a previous scheme based on diagrams is made using the excitation spectra of [PtCl4]2−, CH3I, and H2CO as examples, and the efficiency of the scheme is illustrated.