Theoretical prediction on the structures of the HMgN- and HNMg- anions using multiconfigurational methods

The nine-valence-electron HMgN- and HNMg- anions have been investigated for the first time theoretically using CASSCF (complete active space self-consistent field) and CASPT2 (multiconfiguration second-order perturbation theory) methods in conjunction with the contracted atomic natural orbital (ANO) basis sets. The structures of the low-lying electronic states of HMgN- and HNMg- were predicted. The possible unimolecular conversions between HMgN- and HNMg-were discussed. The calculated results indicated that the ground-state of HMgN-is linear, while the ground-state HNMg- is bent, which is in contradiction to Walsh?s rules predicting linear structures for the HXY systems containing 10 or less valence electrons.

A CASSCF/CASPT2 STUDY ON THE LOW-LYING ELECTRONIC STATES OF THE (CH3)2CHS AND ITS CATION

The multireference approach (CASSCF/CASPT2) combined with the contracted with atomic natural orbital (ANO-RCC-VTZP) basis set has been used to investigate systematically, the low-lying electronic states of ( CH 3)2 CHS in Cs symmetry. The result of geometry optimization using CASSCF/ANO-RCC-VTZP shows that the theoretically determined geometric parameters and harmonic vibrational frequencies for the ground state X2A′ of ( CH 3)2 CHS are in good agreement with previous studies. In addition, we also explored several cationic states adiabatically and found that the 11A′ state of ( CH 3)2 CHS + is unstable and converts to ( CH 3)2 CSH +. The vertical and adiabatic ionization energies were obtained to compare with photoelectron spectroscopic data.

THEORETICAL STUDY ON HBeP- AND HPBe- ANIONS USING MULTICONFIGURATION SECOND-ORDER PERTURBATION THEORY

Some low-lying states of the nine-valence-electron systems HBeP - and HPBe - anions have been studied for the first time using three methods CASSCF, CASPT2 and B3LYP with the contracted atomic natural orbital (ANO) and cc-pVTZ basis sets. The geometries of all stationary points along the potential energy surfaces were optimized at the CASSCF/ANO, CASPT2/ANO and B3LYP/cc-pVTZ levels. The potential energy curves of isomerization reactions between HBeP - and HPBe - were calculated as a function of HBeP bond angle. The ground and the first excited states of HBeP - are predicted to be X2Π and A2Σ+ states, respectively. The X2Σ+ and A2Π states of the linear HPBe - are both first-order saddle points because they have unique imaginary frequency. Two bent minima M1 and M2 were found along the 12A′ and 12A″ potential energy surfaces, respectively. The calculated results indicated that the ground-state HBeP - is linear, while the ground-state HPBe - is bent.

The Ground State Potential for the Chromium Dimer Revisited

Results are presented from a new theoretical study of the ground state potential curve of the chromium dimer using multiconfiguration second-order perturbation theory. A new basis set of the atomic natural orbital type is used where the construction includes correlation of the semi-core 3p orbitals and scalar relativistic effects are added using the Douglas-Kroll Hamiltonian. The active space used in the CASSCF/CASPT2 calculations comprised 16 orbitals with 12 active electrons. The resulting ground state potential is in agreement with experiment. Computed spectroscopic constants are (with experimental values within parentheses): Re = 1.66 (1.68) Å, D0 = 1.65 (1.53 ± 0.06) eV, ∆G1/2 = 413 (452) cm-1. Higher vibrational frequencies are also well reproduced.

Singlet-Triplet Splitting in Methylene: An Accurate Description of Dynamic and Nondynamic Correlation by Reduced Multireference Coupled Cluster Method

A classical multireference problem - the singlet-triplet separation in methylene - is examined by the recently introduced reduced multireference (RMR) singles and doubles coupled cluster (CCSD) method, using both double zeta plus polarization (DZP) and large atomic natural orbital (ANO) basis sets. In the former case, the performance of the RMR CCSD as well as of other approaches is assessed by a comparison with the full configuration interaction (FCI) result that represents the exact solution for this basis, while in the latter case a comparison is made with the experiment. It is shown that using a minimal two-configuration reference space, the RMR CCSD result compares well with either FCI or experiment; and is of the same quality as that provided by the two-reference state universal MR CCSD theory. Both MR CCSD approaches give a balanced description for the singlet and triplet states involved and correct the shortcomings of the single reference CCSD approach that is lacking in the presence of nondynamical correlation effects.