Bingbing Suo

New schemes for internally contracted multi-reference configuration interaction

In this work we present a new internally contracted multi-reference configuration interaction (MRCI) scheme by applying the graphical unitary group approach and the hole-particle symmetry. The latter allows a Distinct Row Table (DRT) to split into a number of sub-DRTs in the active space. In the new scheme a contraction is defined as a linear combination of arcs within a sub-DRT, and connected to the head and tail of the DRT through up-steps and down-steps to generate internally contracted configuration functions. The new scheme deals with the closed-shell (hole) orbitals and external orbitals in the same manner and thus greatly simplifies calculations of coupling coefficients and CI matrix elements. As a result, the number of internal orbitals is no longer a bottleneck of MRCI calculations. The validity and efficiency of the new ic-MRCI code are tested by comparing with the corresponding WK code of the MOLPRO package. The energies obtained from the two codes are essentially identical, and the computational efficiencies of the two codes have their own advantages.

New implementation of the configuration-based multi-reference second order perturbation theory

We present an improved version of the configuration-based multi-reference second-order perturbation approach (CB-MRPT2) according to the formulation of Lindgren on perturbation theory of a degenerate model space. This version involves a reclassification of the perturbation functions and new algorithms to calculate matrix elements in the perturber energy expressions utilizing the graphical unitary group approach and the hole-particle symmetry. The diagonalize-then-perturb (DP), including Rayleigh-Schrödinger and Brillouin-Wigner, and diagonalize-then-perturb-then-diagonalize (DPD) modes have been implemented. The new CB-MRPT2 method is applied to several typical and interesting systems: (1) the vertical excitation energies for several states of CO and N(2), (2) energy comparison and timing of the ground state of C(4)H(6), (3) the quasi-degeneracy of states in LiF, (4) the intruder state problems of AgH, and (5) the relative energies of di-copper-oxygen-ammonia complex isomers. The results indicate that the computational accuracy and efficiency of the presented methods are competitive and intruder-free. It should be emphasized that the DPD method rectifies naturally the shortcomings of LiF potential energy curves constructed by the original second order complete active space perturbation theory (CASPT2), without having to recourse to the so-called state mixture. Unlike CASPT2, the new methods give the same energy ordering for the two di-copper-oxygen-ammonia isomers as the previous multi-reference configuration interaction with single and double excitations methods. The new CB-MRPT2 method is shown to be a useful tool to study small to medium-sized systems.

Performance of TD-DFT for Excited States of Open-Shell Transition Metal Compounds

Time-dependent density functional theory (TD-DFT) has been very successful in accessing low-lying excited states of closed-shell systems. However, it is much less so for excited states of open-shell systems: unrestricted Kohn-Sham based TD-DFT (U-TD-DFT) often produces physically meaningless excited states due to heavy spin contaminations, whereas restricted Kohn-Sham based TD-DFT often misses those states of lower energies. A much better variant is the explicitly spin-adapted TD-DFT (X-TD-DFT) [J. Chem. Phys. 2011, 135, 194106] that can capture all the spin-adapted singly excited states yet without computational overhead over U-TD-DFT. While the superiority of X-TD-DFT over U-TD-DFT has been demonstrated for open-shell systems of main group elements, it remains to be seen if this is also the case for open-shell transition metal compounds. Taking as benchmark the results by MS-CASPT2 (multistate complete active space second-order perturbation theory) and ic-MRCISD (internally contracted multireference configuration interaction with singles and doubles), it is shown that X-TD-DFT is indeed superior to U-TD-DFT for the vertical excitation energies of ZnH, CdH, ScH2, YH2, YO, and NbO2. Admittedly, there exist a few cases where U-TD-DFT appears to be better than X-TD-DFT. However, this is due to a wrong reason: the underestimation (due to spin contamination) and the overestimation (due to either the exchange-correlation functional itself or the adiabatic approximation to the exchange-correlation kernel) happen to be compensated in the case of U-TD-DFT. As for [Cu(C6H6)2]2+, which goes beyond the capability of both MS-CASPT2 and ic-MRCISD, X-TD-DFT revises the U-TD-DFT assignment of the experimental spectrum.

New implementations of MRCI in semiempirical frameworks

Multireference configuration interaction with single and double excitations (MRCISD) as well as its analytic CI gradients has been implemented in the semiempirical framework. The hole‐particle symmetry and a mixed driven model for computing coupling coefficients have been used in the new code that allows us to perform MRCI and gradient calculations with higher efficiency and less storage requirements.

Potential energy curves and interpretation of electronic spectrum of the rhodium monoxide

Potential energy curves of 17 electronic states of rhodium monoxide (RhO) are calculated by multireference configuration interaction with single and double excitations (MRCISD). The ground state of RhO is determined to be a (4)Sigma(-) state with equilibrium bond length of 1.710 A and harmonic vibrational frequency of 825 cm(-1) at the MRCISD level of theory. It dissociates into Rh((4)F)+O((3)P) with a dissociation energy of 3.77/4.26 eV (MRCISD/MRCISD+Q), which is in agreement with the experimental value of 4.19+/-0.43 eV. Two low-lying excited states a (2)Sigma(-) and b(2)Pi are located at 4152 and 7154 cm(-1) above the ground state. The b(2)Pi with the adjacent (2)Delta, (4)Delta, and (2)Pi(II) states can be strongly coupled via spin-orbit interaction leading to a large splitting between b (2)Pi(3/2)-b (2)Pi(1/2) states with the value of 2422 cm(-1), which is comparable with the experimental value of 2400 cm(-1). Two higher doublets, c(2)Pi and d(2)Pi, have the same dominant configuration, 10sigma(2)11sigma(2)12sigma(1)5pi(4)6pi(3)2delta(3), and their transitions to the ground state, i.e., c(2)Pi-->(4)Sigma(-) and d(2)Pi-->(4)Sigma(-), correspond to the two visible bands of RhO.

Theoretical Study of Low-Lying Ω Electronic States of PtH and PtH+

Potential energy curves of 65 and 147 low-lying Ω states of PtH and PtH+ are respectively constructed using the multireference configuration interaction with singles, doubles, and Davidsons cluster corrections (MRCISD+Q), and the spin-orbit coupling effects are considered through the state-interaction approach with relativistic effective core potential spin-orbit operators. Spectroscopic constants fitted from these curves are reported and are compared with the available experimental or theoretical values. With the aid of the theoretical results including transition dipole moments, some experimentally reported electronic states and spectral bands of PtH are analyzed and reassigned. This work provides useful reference data for future experimental and theoretical studies of PtH and PtH+.

Finite-field calculation of static polarizabilities and hyperpolarizabilities of In + and Sr

The finite field calculations are performed for two heavy frequency-standard candidates In

Finite-field calculation of the polarizabilities and hyperpolarizabilities of Al+

^+

Electronic Structure of OsSi Calculated by MS-NEVPT2 with Inclusion of the Relativistic Effects

and Sr. The progressive hierarchy of electron correlations is implemented by the relativistic coupled-cluster and configuration interaction methods combined with basis set of increasing size. The dipole polarizabilities, dipole hyperpolarizabilities, quadrupole moments, and quadrupole polarizabilities are recommended for the ground state 5s

Graphical unitary group approach based on hole-particle symmetry using in multi-reference configuration interaction

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