Potential energy curve for isomerization of N2H2 and C2H4 using the improved virtual orbital multireference Møller–Plesset perturbation theory

Abstract

Multireference Møller-Plesset (MRMP) perturbation theory [K. Hirao, Chem. Phys. Lett. 190, 374 (1992)] is modified to use improved virtual orbitals (IVOs) and is applied to study ground state potential energy curves for isomerization and dissociation of the N2H2 and C2H4 molecules. In contrast to traditional MRMP or multistate multiconfiguration quasidegenerate perturbation theory where the reference functions are obtained from (often difficult to converge) state averaged multiconfiguration self-consistent field methods, our reference functions are represented in terms of computationally efficient IVOs. For convenience in comparisons with other methods, a first order complete active space configuration interaction (CASCI) calculation with the IVOs is followed by the use of the IVOs in MRMP to incorporate residual electron correlation effects. The potential energy curves calculated from the IVO-MRMP method are compared with computations using state-of-the-art coupled cluster singles and doubles (CCSD) methods and variants thereof to assess the efficacy of the IVO-MRMP scheme. The present study clearly demonstrates that unlike the CCSD and its variants, the IVO-MRMP approach provides smooth and reliable ground state potential energy curves for isomerization of these systems. Although the rigorously size-extensive completely renormalized CC theory with noniterative triples corrections (CR-CC(2,3)) likewise provides relatively smooth curves, the CR-CC(2,3) calculations overestimate the cis-trans barrier height for N2H2. The ground state spectroscopic constants predicted by the IVO-CASCI method agree well with experiment and with other highly correlated ab initio methods.

Molecular applications of analytical gradient approach for the improved virtual orbital-complete active space configuration interaction method

“…34͒ methods to append dynamical correlation into the IVO-CASCI energy and wave function. These recent studies [36][37][38][39] further demonstrate that the improved virtual orbital multireference Möller-Plesset ͑IVO-MRMP͒/MCQDPT approaches offer very promising tools for investigating geometries and potential energy surfaces for electronic states that are strongly perturbed by intruders and/or that have pronounced multireference character. Another approach for skipping the expensive orbital optimization step is the multireference Möller-Plesset complete active space configuration interaction method of Hirao and co-workers 29 that involves optimization of only the expansion coefficients of the full configuration interaction ͑CI͒ configurations in an active space.…”

“…Recent calculations by Chaudhuri et al 36 combine the IVO-CASCI approach with multireference Möller-Plesset perturbation theory 33 and MCQDPT ͑Ref. 34͒ methods to append dynamical correlation into the IVO-CASCI energy and wave function.…”

Application of an efficient multireference approach to free-base porphin and metalloporphyrins: Ground, excited, and positive ion states

“…[52][53][54][55][56][57][58][59] and IVO-CASCI based multireference Möller-Plesset (IVO-MRMP) perturbation theory 60, 61 methods to determine the ground and excited state geometries and vertical excitation energies (EEs) and ionization potentials (IPs) of free-base porphin (H 2 P where P stands for porphyrin ring) and its Mg and Zn metal derivatives (termed as MgP and ZnP, respectively). The approach is based on the MRMP version of Hirao et al 62 and yields similar or better results compared to MRMP (with the same reference function and orbitals) at low computational expense.…”

Ab Initio Multireference Investigation of Disjoint Diradicals: Singlet versus Triplet Ground States

“…The IVO-CASCI-based MRPT method has been successfully applied to many chemical problems, and appears as a useful technique for treating nondynamic and dynamic correlation effects in a balanced and accurate manner. [19] The IVO-based methods have several attractive features (which support our use of these methods for treating the TME system) including 1) size-consistency, 2) applicability to ground and excited states (in a single framework) with closed or open shells for the entire energy surface (provided the CAS is correctly selected), 3) general applicability to a wide class of problems and molecules, and 4) cost effectiveness (less expensive than the corresponding CASSCF-based MRMPPT [20] /MCQDPT [21] methods). The IVO-MRMPPT (or IVO-MCQDPT) methods are comparable in accuracy to the MRMPPT (or MCQDPT) CASSCF approaches and ameliorate (but do not eliminate) the sharp increase in complexity of the MRMPPT or MCQDPT methods as the number of active orbitals and determinantal configurations increase.…”

“…TME thus represents an excellent probing ground for establishing the generality of IVO-based methods that are tailored to account for electroncorrelation effects ranging from full degeneracy (in the singlet conformation) to nondegeneracy (in the triplet case considered here). Herein, we employed improved virtual orbital CAS configuration interaction (IVO-CASCI) [18] based multireference perturbation theory [coined as IVO-MRMPPT (improved virtual orbital multireference Møller-Plesset perturbation theory) or IVO-MCQDPT (improved virtual orbital multiconfigurational quasidegenerate perturbation theory)] [19] to study the lowest-energy singlet and triple states of disjoint diradicals, for example TME. The IVO-CASCI-based MRPT method has been successfully applied to many chemical problems, and appears as a useful technique for treating nondynamic and dynamic correlation effects in a balanced and accurate manner.…”