Multireference Internally Contracted Configuration Interaction Research Articles

Accurate transport properties for O(3P)-H and O(3P)-H2.

Transport properties for collisions of oxygen atoms with hydrogen atoms and hydrogen molecules have been computed by means of time-independent quantum scattering calculations. For the O(3P)-H(2S) interaction, potential energy curves for the four OH electronic states emanating from this asymptote were computed by the internally-contracted multi-reference configuration interaction method, and the R-dependent spin-orbit matrix elements were taken from Parlant and Yarkony [J. Chem. Phys. 110, 363 (1999)]. For the O(3P)-H2 interaction, diabatic potential energy surfaces were derived from internally contracted multi-reference configuration interaction calculations. Transport properties were computed for these two collision pairs and compared with those obtained with the conventional approach that employs isotropic Lennard-Jones (12-6) potentials.

Ab initio potential energy surfaces for the 1A′ and 3A′ states of the MgH(X 2Σ +) + H( 2S) system

The two potential energy surfaces (PESs) associated to the 1 A′ and 3 A′ states correlating to the interacting MgH(X 2Σ +) + H( 2S) system were determined from ab initio calculations. They were obtained using the aug-cc-pVQZ basis set for the H atom and the cc-pV5Z basis set for the Mg atom and multireference internally contracted configuration-interaction (MRCI) method including Davidson correction. The MgH distance was kept fixed at its experimental equilibrium distance in the X 2Σ ground state. The ab initio calculated interaction energies of the two PESs were fitted analytically on the basis of Legendre polynomials for future collisional rotational excitation study. The 1 A′ and 3 A′ adiabats both present very deep well corresponding to the MgH 2 complex.

Characterization of the HSiNHNSi system in its electronic ground state

The electronic ground states (X (1)Sigma(+)) of HSiN, HNSi, and the transition state connecting the two isomers were systematically studied using configuration interaction with single and double (CISD) excitations, coupled cluster with single and double (CCSD) excitations, CCSD with perturbative triple corrections [CCSD(T)], multireference complete active space self-consistent field (CASSCF), and internally contracted multireference configuration interaction (ICMRCI) methods. The correlation-consistent polarized valence (cc-pVXZ), augmented correlation-consistent polarized valence (aug-cc-pVXZ) (X=T,Q,5), correlation-consistent polarized core-valence (cc-pCVYZ), and augmented correlation-consistent polarized core-valence (aug-cc-pCVYZ) (Y=T,Q) basis sets were used. Via focal point analyses, we confirmed the HNSi isomer as the global minimum on the ground state HSiN_HNSi zero-point vibrational energy corrected surface and is predicted to lie 64.7 kcal mol(-1) (22 640 cm(-1), 2.81 eV) below the HSiN isomer. The barrier height for the forward isomerization reaction (HSiN-->HNSi) is predicted to be 9.7 kcal mol(-1), while the barrier height for the reverse process (HNSi-->HSiN) is determined to be 74.4 kcal mol(-1). The dipole moments of the HSiN and HNSi isomers are predicted to be 4.36 and 0.26 D, respectively. The theoretical vibrational isotopic shifts for the HSiN/DSiN and HNSi/DNSi isotopomers are in strong agreement with the available experimental values. The dissociation energy for HSiN [HSiN(X (1)Sigma(+))-->H((2)S)+SiN(X (2)Sigma(+))] is predicted to be D(0)=59.6 kcal mol(-1), whereas the dissociation energy for HNSi [HNSi(X (1)Sigma(+))-->H((2)S)+NSi(X (2)Sigma(+))] is predicted to be D(0)=125.0 kcal mol(-1) at the CCSD(T)/aug-cc-pCVQZ level of theory. Anharmonic vibrational frequencies computed using second order vibrational perturbation theory are in good agreement with available matrix isolation experimental data for both HSiN and HNSi isomers root mean squared derivation (RMSD=9 cm(-1)).

CCSD(T) and MRCI studies on the ground and excited states of BrOClO and ClOBrO

AbstractIn this work, three forms (cis, trans and nonplanar) of ClOBrO and BrOClO were optimized at CCSD(T)/cc‐pVTZ level of theory. At the most stable forms (nonplanar form) of ClOBrO and BrOClO, the vertical excitation energies for the lowest six singlet states and two triplet states were calculated at the multireference internally contracted configuration interaction (MRCI) level of theory using cc‐pVDZ, Aug‐cc‐pVDZ, cc‐pVTZ, and Aug‐cc‐pVTZ basis sets. The scalar relativistic effect on the excited states of BrOClO and ClOBrO were estimated. In addition, the potential energy curves of the lowest six singlet states and two triplet states of BrOClO and ClOBrO, as well as BrOOCl were calculated at both MCSCF (complete active space self‐consistent field) and MRCI levels of theory using Aug‐cc‐pVDZ basis set on the active space (18e,12o) along the distances of BrOClO, ClOBrO, and BrOOCl. The results were compared among BrOOCl, ClOBrO, and BrOClO. The first singlet excited state of BrOOCl is 1.12 eV higher than that of BrOClO and 1.36 eV higher than that of ClOBrO at MRCI/cc‐pVTZ level of theory. The first triplet excited state of BrOOCl is 0.77 eV higher than that of BrOClO and 0.86 eV higher than that of ClOBrO at MRCI/cc‐pVTZ level of theory. Most of the excited states of BrOClO studied in this work are unbound states; but most of the ClOBrO and BrOOCl excited states studied in this work are weakly bound states at MRCI level of theory. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009

Potential energy surface for the [formula omitted] system: Application to the rotationally inelastic scattering of C 2 in collisions with He

A new two-dimensional potential energy surface (2D-PES) for the interaction of He( 1S) with the C 2 ( X 1 Σ g + ) molecule in its ground state is computed using multireference internally contracted configuration-interaction (MRCI) including Davidson correction (+Q) and the augmented correlation-consistent valence quadruple-zeta (aug-cc-pVQZ) basis set of Dunning. The C 2 ( X 1 Σ g + ) bond length was fixed at the experimental equilibrium distance. Rotationally inelastic scattering cross sections are computed using close-coupling (CC) method. The corresponding rate coefficients for the rotational transitions up to j = 20 are calculated for the temperatures 5 ⩽ T ⩽ 300 K.

Ab initio potential energy surfaces for the study of rotationally inelastic CH(X2Π)+H(2S) collisions

Abstract The four potential energy surfaces (PESs) associated, respectively to 1 A′, 3 A′, 1 A″ and 3 A ″ states correlating to the interacting CH ( X 2 Π ) + H ( 2 S ) system, were determined using the aug-cc-pVQZ basis set, and multireference internally contracted configuration-interaction (MRCI) calculation method including the Davidson correction. The CH bond length was fixed at the equilibrium value of the X 2 Π ground state. The PES of the A ′ state considerably differs from that of the A ″ state. The ab initio calculated interaction energies of the A ′ and A ″ PESs for each spin multiplicity were fitted analytically on the basis of Legendre polynomials.

Born−Oppenheimer Symmetry Breaking in the C̃ State of NO2: Importance of Static and Dynamic Correlation Effects

A systematic theoretical treatment is performed with highly correlated ab initio theoretical methods to establish the structural nature of the C state of NO2. We predict the C state to have an asymmetric structure (point group C(s)). Spin-restricted and spin-unrestricted configuration interaction (CISD), coupled cluster [CCSD and CCSD(T)], multireference complete active space self-consistent field (CASSCF), and internally contracted multireference configuration interaction (ICMRCI) methods were used in conjunction with very large correlation-consistent polarized valence zeta cc-pVXZ and aug-cc-pVXZ [X = T, Q, 5] basis sets. The asymmetric C 2A'' state is predicted to lie T(e) = 47.5 kcal/mol (2.06 eV, 16,600 cm(-1)) above the X 2A1 state at the aug-cc-pV5Z/UCCSD(T) level of theory, with T0 = 46.0 kcal/mol (2.00 eV, 16,100 cm(-1)), in good agreement with the experimental values of 46.77 kcal/mol (2.028 eV, 16,360 cm(-1)) by Weaver and 46.42 kcal/mol (2.013 eV, 16,234 cm(-1)) by Aoki. The symmetric structure (in C(2v) symmetry) with re(NO) = 1.274 A and theta(e) (ONO) = 109.9 degrees is a transition state between the two equivalent asymmetric (in C(s) symmetry) structures and is located only 1.53 kcal/mol (0.066 eV, 540 cm(-1)) above the asymmetric structure. The asymmetric structure is predicted to have structural parameters r(e)(NOl) = 1.489 A, r(e)(NOs) = 1.169 A, and theta(e)(ONO) = 109.7 degrees with the same method, aug-cc-pV5Z/UCCSD(T). The averaged NO bond distance is 1.329 A, and the difference between the two NO bond distances is 0.320 A. The three harmonic vibrational frequencies for the C 2A'' state are 1656 (in-phase stretch), 759 (bend), and 378 (out- of-phase stretch) cm(-1). While these theoretical results further corroborate the previous predictions concerning the asymmetric nature of the C state, there remains discrepancy between the theoretical and experimental symmetric stretching mode omega1 (1656 and 923 cm(-1), respectively). It is possible, however, that this disagreement could be resolved by a reassignment of the corresponding lines in the experimental spectrum, though additional vibronic simulations of the spectrum are required to confirm this proposition.

Vibrational energy levels for the electronic ground state of the diazocarbene (CNN) molecule

The vibrational energy levels of diazocarbene (diazomethylene) in its electronic ground state, CNN, have been predicted using the variational method. The potential energy surfaces of CNN were determined by employing ab initio single reference coupled cluster with single and double excitations (CCSD), CCSD with perturbative triple excitations [CCSD(T)], multi-reference complete active space self-consistent-field (CASSCF), and internally contracted multi-reference configuration interaction (ICMRCI) methods. The correlation-consistent polarised valence quadruple zeta (cc-pVQZ) basis set was used. Four sets of vibrational energy levels determined from the four distinct analytical potential functions have been compared with the experimental values from the laser-induced fluorescence measurements of Wurfel et al. obtained in 1992. The CCSD, CCSD(T), and CASSCF potentials have not provided satisfactory agreement with the experimental observations. In this light, the importance of both non-dynamic (static) and dynamic correlation effects in describing the ground state of CNN is emphasised. Our best theoretical fundamental frequencies at the cc-pVQZ ICMRCI level of theory, ν1 = 1230, ν2 = 394, and ν3 = 1420 cm− 1, are in excellent agreement with the experimental values of ν1 = 1235, ν2 = 396, and ν3 = 1419 cm− 1, and the mean absolute deviation between the 23 calculated and experimental vibrational energy levels is only 7.4 cm− 1. It is shown that the previously suggested observation of the ν3 frequency at about 2847 cm− 1 was in fact the first overtone 2ν3.

Electronic States of the Ultramarine Chromophore S3 -

The potential energy surfaces and the spin–orbit couplings for states correlating with the lowest dissociation asymptote S2(X 3 Σ - g ) + S-(2 P) of the radical anion of thiozone have been calculated by full valence complete active space self-consistent field (CASSCF), internally contracted multi-reference configuration interaction (IC-MRCI) and for some of the states also by restricted coupled cluster with perturbative triples (RCCSD(T)) ab initio methods. For six electronic states lying below the electron detachment threshold: X 2 B 1, A 2 B 2, B 2 A 1, C 2 A 2, 4 Σ g -, and the second 2 A 1, equilibrium geometries and harmonic wavenumbers are reported. For the ground state also anharmonic vibrational levels have been evaluated. Due to the intensity borrowing via vibronic coupling with the B 2 A 1 state also the dipole forbidden A–X transition becomes allowed in the Franck–Condon region of the UV spectrum. The C 2 A 2 state responsible for the blue color of ultramarine correlates adiabatically with the first dissociation asymptote S2(X 3 Σ - g ) + S-(2 P). Along the dissociation path the regions of vibronic and spin–orbit couplings were located. For the R S2 - S larger than about 2.6Å the electron spin-states of the lowest dissociation asymptote are strongly mixed. The properties of the S3 - electronic states are compared with those of O3 -.

Ab initio calculations on the X̃A′1 and ÃA″1 states of HPO and Franck-Condon simulation of the single vibronic level emission spectra of HPO and DPO

Minimum-energy geometries and relative electronic energies of the X (1)A(') and A (1)A(") states of HPO have been computed employing the coupled-cluster single-double plus perturbative triple excitations {RCCSD(T)} and/or complete-active-space self-consistent-field (CASSCF) multireference internally contracted configuration interaction (MRCI) methods with basis sets of up to the augmented correlation-consistent polarized-valence quintuple-zeta (aug-cc-pV5Z) quality. In addition, RCCSD(T)/aug-cc-pVQZ and CASSCF/MRCI/aug-cc-pVQZ potential energy functions, anharmonic vibrational wave functions, and energies involving all three vibrational modes for both electronic states of HPO and DPO, and Franck-Condon factors between the two electronic states, which allow for Duschinsky rotation and anharmonicity, were computed. Computed Franck-Condon factors were then used to simulate single vibronic level (SVL) emission spectra recently reported by Tackett and Clouthier [J. Chem. Phys. 117, 10604 (2002)]. Excellent agreement between the simulated and observed spectra was obtained for the A (1)A(")(1,0,0)-->X (1)A(') SVL emission of HPO and DPO, when the best estimated ab initio geometries of the two states, which include contributions from core correlation and extrapolation to the complete basis set limit, were used in the simulation, suggesting that the best estimated ab initio geometry of the A (1)A(") state of HPO, particularly the bond angle of 94.5 degrees , is more reliable than the available experimentally derived geometry. A discussion on the geometrical parameters derived from rotational constants obtained from the rotational analysis of a high-resolution spectrum and from Franck-Condon simulation of the vibrational structure of an electronic spectrum is given.

Multiconfigurational self-consistent field and multireference internally contracted configuration interaction studies on the excited states of weakly bonded NO2 dimer (N2O4)

In this paper, the vertical excitation energies of total of 32 states of N(2)O(4) including the lowest two singlet states and two triplet states of each of the A(g), B(3u), B(2u), B(1g), B(1u), B(2g), B(3g), and A(u) symmetries were calculated at multiconfigurational self-consistent field (MCSCF) and the multireference internally contracted configuration interaction (MRCI) levels of theory on the active space (15o,16e) with aug-cc-pVDZ basis set. The potential energy curves of the eight singlet states(1 (1)A(g), 1 (1)B(3u), 1 (1)B(2u), 1 (1)B(1g), 1 (1)B(1u), 1 (1)B(2g), 1 (1)B(3g), and 1 (1)A(u)) and eight triplet states (1 (3)A(g), 1 (3)B(3u), 1 (3)B(2u), 1 (3)B(1g), 1 (3)B(1u), 1 (3)B(2g), 1 (3)B(3g), and 1 (3)A(u)) were calculated at MCSCF and MRCI levels of theory on the active space (15o,16e) with aug-cc-pVDZ basis set along the N-N distance. The vertical excitation energies of 1 (1)B(3u), 1 (1)B(2u), and 1 (1)B(1u) states with nonzero transition moment are 4.60 eV (269.6 nm), 6.06 eV (204.6 nm), and 7.71 eV (160.8 nm), respectively, at MRCI level of theory. The photodissociation asymptotics were assigned as NO(2)(X (2)A(1))+NO(2)(X (2)A(1)) for ground state 1 (1)A(g) and the 1 (3)B(1u) state, NO(2)(X (2)A(1))+NO(2)(1 (2)A(2)) for the 1 (1)B(1g), 1 (3)B(1g), 1 (1)A(u), and 1 (3)A(u) states, NO(2)(X (2)A(1))+NO(2)(1 (2)B(1)) for the 1 (1)B(3u), 1 (3)B(3u), 1 (1)B(2g), and 1 (3)B(2g) states, and NO(2)(X (2)A(1))+NO(2)(1 (2)B(2)) for the 1 (1)B(2u), 1 (3)B(2u), 1 (1)B(3g), and 1 (3)B(3g) states.

Low-lying quartet electronic states of nitrogen dioxide

The environmentally active molecule nitrogen dioxide (NO2) has been systematically studied using high level theoretical methods. The electronic ground state and the low-lying quartet states of NO2 have been investigated. Single reference restricted open-shell self-consistent field (SCF), complete active space SCF (CASSCF), spin-restricted (R) and spin-unrestricted (U) configuration interaction with single and double excitations (CISD), coupled cluster with single and double excitations (CCSD), CCSD with perturbative triple excitations [CCSD(T)], and internally contracted multireference configuration interaction (ICMRCI) methods along with Dunning's correlation consistent polarized valence cc-pVXZ and augmented cc-pVXZ (where X=T,Q,5) basis sets were used in this research. At the aug-cc-pV5Z/UCCSD(T) level the classical adiabatic excitation energies (Te values) of the three lowest-lying quartet excited states were predicted to be 83.3 kcalmol (3.61 eV, 29 200 cm(-1)) for the ã 4A2 state, 93.3 kcalmol (4.05 eV, 32 600 cm(-1)) for the b 4B2 state, and 100.8 kcalmol (4.37 eV, 35 300 cm(-1)) for the c 4A1 state. The quantum mechanical excitation energies (T 0 values) were determined to be 81.6 kcalmol (3.54 eV, 28 500 cm(-1)) for the a 4A2 state and 90.7 kcalmol (3.93 eV, 31 700 cm(-1)) for the b 4B2 state. The lowest quartet linear Renner-Teller 4Pi state gives rise to the a 4A2 state with 112.8 degrees and the b 4B2 state with 124.4 degrees <(ONO) bond angles upon bending. The b state shows some peculiar behavior. Although CASSCF, RCISD, UCISD, RCCSD, UCCSD, and RCCSD(T) methods predicted the presence of a Cs equilibrium geometry (a double minimum 4A' state), SCF, UCCSD(T), and ICMRCI wave functions predicted the C2v structure for the b 4B2 state. The importance of both dynamical and nondynamical correlation treatments for the energy difference between C2v and Cs structures of b state is highlighted in this context. The c 4A1 state is predicted to have a very small bond angle of 85.8 degrees . Potential energy diagrams with respect to the bond angles of the ground state and four quartet states are presented.

Computational studies on the ground and excited states of BrOOBr

In this work, theoretical computations for the ground and excited states of BrOOBr have been performed at high-level ab initio molecular orbital theories. The ground-state geometries of BrOOBr in different forms (trans, cis, and twist form) have been optimized at the couple-cluster CCSD(T) level of theory with cc-pVTZ and aug-cc-pVTZ basis sets, which indicates that at CCSD(T)/cc-pVTZ level of theory, the twist form is 4.96 kcal/mol more stable than the trans form and 10.67 kcal/mol more stable than the cis form; at the CCSD(T)/aug-cc-pVTZ basis set the twist form is 4.33 kcal/mol more stable than the trans form and 9.54 kcal/mol more stable than the cis form. The vertical excitation energies and potential-energy curves for the singlet and triplet low-lying excited states of BrOOBr were calculated at both the complete active space self-consistent-field (CASSCF) level of theory and the multireference internally contracted configuration interaction (MRCI) level of theory. The differences of potential-energy curves at CASSCF and MRCI levels of theory are found for the BrOOBr excited states. At CASSCF level of theory, none of the BrOOBr excited states are bound. However, at MRCI level of theory, all the BrOOBr states studied in this work are bound or slightly bound at the Frank-Condon region. In addition, the scalar relativistic effect and the spin-orbital coupling effect on the vertical excitation energies of the electronic states of BrOOBr were estimated.

Potential energy functions of the X̃ 2B1, Ã 2B2, B̃ 2A1, and C̃ 2A2 states of Cl2O+ and the X̃ 1A1 state of Cl2O: Franck–Condon simulations of photoelectron bands of Cl2O which include anharmonicity

Restricted-spin coupled-cluster single and double plus perturbative triple excitations [RCCSD (T)] and/or complete-active-space self-consistent-field multireference internally-contracted configuration interaction (CASSCF/MRCI) potential energy functions of the X̃ 2B1, à 2B2, B̃ 2A1, and C̃ 2A2 states of Cl2O+ and the X̃ 1A1 state of Cl2O, with basis sets of up to the augmented-correlation-consistent-polarized-valence-quadruple-zeta quality, have been reported. For each of these states, vibrational wave functions of the symmetric stretching and bending modes have been computed, employing the potential energy function obtained at the highest level of calculation, with Watson’s Hamiltonian and anharmonic vibrational wave functions expressed as linear combinations of harmonic basis functions. The helium I photoelectron spectrum of Cl2O has been simulated with Franck–Condon factors calculated using computed anharmonic vibrational wave functions and allowing for Duschinsky rotation. The adiabatic ionization energies (AIEs) to the four lowest cationic states of Cl2O+ have been evaluated at the RCCSD(T) level with basis sets of up to polarized-valence-quintuple-zeta quality and by various extrapolation techniques to the basis set limit. Revised equilibrium geometrical parameters of the X̃ 2B1 and C̃ 2A2 states of Cl2O+ were obtained from the iterative Franck–Condon analysis procedure, and revised AIEs for the à 2B2 and B̃ 2A1 states of Cl2O+ were estimated based on comparison between the simulated and observed photoelectron spectra. It was found that inclusion of anharmonicity in the Franck–Condon factor calculations for each electronic state improves the quality of the simulated spectrum. The computed T1 diagnostics from the RCCSD calculations suggest that the B̃ 2A1 state of Cl2O+, with the ⋯(9a1)1(3b2)2(10a1)0 electronic configuration, possesses multiconfigurational character in the region of r(ClO)=1.87 Å and θ(ClOCl)=125°. CASSCF/MRCI/aug-cc-pVQZ(no g) calculations show an avoided crossing between the B̃ 2A1 state and (2)2A1 state [with the ⋯(9a1)2(3b1)0(10a1)1 electronic configuration], in the region of 1.96&amp;gt;r&amp;gt;1.80 Å and 137.0&amp;gt;θ&amp;gt;132.0°.

CASSCF and MRCI studies of the electronic excited states of CH2Cl and CH2Br

The potential energy curves for the low-lying excited states of CH2Cl and CH2Br have been computed using the complete active space self-consistent field (CASSCF) with the cc-pVTZ basis sets. The vertical excitation energies for the excited states were calculated using the multireference internally contracted configuration interaction (MRCI) method with the cc-pVTZ basis set for CH2Cl, and cc-pVTZ and ECP basis sets for CH2Br. For CH2Cl and CH2Br, all the excited states studied are found to be repulsive along the C–Cl and C–Br coordinate.

High level ab initio molecular orbital theory study of the structure, vibrational spectrum, stability, and low-lying excited states of HOONO

Ab initio molecular orbital methods have been employed to determine the molecular structure, vibrational frequencies, and stability of HOONO. These parameters were determined using quadratic configuration interaction methods with Dunning’s correlation consistent basis sets. Low-lying excited states for HOONO have been determined using complete active space self-consistent field (CASSCF) and multireference internally contracted configuration interaction (MRCI) methods. The first excited state (1 1A″) is calculated to be 4.19 eV above the ground state of HOONO. Potential energy curves for the ground and excited states are presented.

CASSCF and MRCI studies on the electronic excited states of CHBrO, CClBrO, and CBr2O

The potential energy curves for the low-lying excited states of CHBrO, CClBrO, and CBr2O have been computed using the complete active space self-consistent field (CASSCF) method. The vertical excitation energies for the excited states were calculated using the multireference internally contracted configuration interaction (MRCI) method with the cc-pVTZ and cc-pVTZ+spdf diffuse functions basis sets. The vertical excitation energies were also calculated with an ECP basis set. Results show that the first excited states for CHBrO, CClBrO, and CBr2O are slightly bound in the CX (X=H, Cl, and Br) coordinates. However, the first excited state exhibits predissociation (to ground state products) as a result of curve-crossing. Dissociation of CHBrO should result in Br atoms and HCO radicals, while in CClBrO, there are two competing channels for dissociation, namely, Cl+Br+CO and Br+ClCO. In CBr2O, photodissociation is predicted to result in the production of 2Br+CO.

Surface aligned photochemistry: Photodissociation of Cl2 and Cl2⋯Cl adsorbed on LiF(001)

Photodissociation of chlorine adsorbed on a LiF(001) surface at 25–70 K has been investigated by means of angularly resolved resonantly enhanced multiphoton ionization spectroscopy (REMPI). The translational-energy distributions and angular distributions for forming Cl(g) photofragments were determined. Photolysis was performed employing 351 nm radiation, with laser pulse energies of 0.3–1.2 mJ/cm2. A peak in the translational energy of Cl(g) at about 0.4 eV was identified as being due to the direct photodissociation of the Cl2(ad) molecule by 3.5 eV photons. Particular interest attached to the observation of a further channel (termed “A”) for photodissociation leading to Cl(g) with translational energy peaking at ∼1 eV and extending to 1.5 eV. The available photon energy renders it highly unlikely that this “high-energy” Cl(g) originates in Cl2(ad). Channel A had the same linear dependence of Cl-atom flux on laser pulse-energy as did the lower energy (0.4 eV) channel, termed “B,” but differed from it in exhibiting a slow approach to steady state. It appears that channel A requires the prior build-up of Cl(ad) concentration due to the photodissociation of Cl2. It is proposed that this leads to the formation of a steady-state concentration of Cl2⋯Cl which when photolyzed yields high-energy Cl(g) via channel A. Channel A exhibits a distinctive angular distribution at low coverage and a characteristic Cl*/Cl ratio, as compared with channel B. The suggested mechanism for channel A is Cl2⋯Cl+hν→Cl3*→Cl2•Cl→Cl2+Cl (where* is an electronically excited state and • represents repulsion in the lower electronic state to which Cl3* reverts). This mechanism is interpreted in terms of an extensive diatomics-in-molecules (DIM) model for the trichlorine radical, shown to be in agreement with high level ab initio multireference internally contracted configuration interaction (MRCI) calculations, and consistent with the observations.

Vibrational resonances of non-rotating HCN in the excited

A multi reference internally contracted configuration interaction (MRCI) method is used to generate the potential energy function (PEF) of the excited electronic state of HCN molecule. The analytic representation of the PEF is employed to calculate complex eigenvalues (resonance positions and widths) by a discrete variable representation (DVR) of the Hamiltonian for the non-rotating (J =0) molecule. The computational method used is a variant of the filter-diagonalization technique based on a recursive polynomial expansion of the absorbing-boundary-conditions (ABC) Green operator. Reasonable agreement with existing experimental data is found.

Improving CISD calculations by geminal-type reference states

The possibility of using geminal-type wavefunctions as multi-configurational reference states for configuration interaction (CI) calculations is investigated. The reference function class considered here includes 2-electron CAS, certain types of MC-SCF and APSG (antisymmetrized product of strongly orthogonal geminals) as special cases. The method is not variational but exact for two electrons. The relation between the developed formalism and GVB-CI, internally contracted multi-reference CI (IC-MRCI) and EOM-CC techniques is discussed. We present applications at the CISD level. The results show that total energies generally fall between standard CISD and CCSD values. The proposed modification of CISD decreases the size-consistency error and eliminates it if the molecule is fragmented into two-electron pieces. Preliminary calculations show an appreciable improvement of CISD dissociation energies.