Application of improved virtual orbital based multireference methods to N2, LiF, and C4H6 systems

Abstract

The improved virtual orbital (IVO) complete active space configuration interaction (CASCI) based multiconfigurational quasidegenerate perturbation theory (MCQDPT) and its single-root version (termed as MRMPPT) are applied to assess the efficacy and the reliability of these two methods. Applications involve the ground and/or excited state potential energy curves (PECs) of N(2), LiF, and C(4)H(6) (butadiene) molecules, systems that are sufficiently complex to assess the applicability of these methods. The ionic-neutral curve crossing involving the lowest two (1)Sigma(+) states of LiF molecule is studied using the IVO-MCQDPT method, while its single-root version (IVO-MRMPPT) is employed to study the ground state PEC for isomerization of butadiene and to model the bond dissociation of N(2) molecule. Comparisons with the standard methods (full CI, coupled cluster with singles and doubles, etc.) demonstrate that the IVO-based MRMPPT and MCQDPT approaches provide smooth and reliable PECs for all the systems studied. The IVO-CASCI method is explored to enable geometry optimization for ground state of C(4)H(6) using numerical energy gradients. The ground spectroscopic constants of N(2) and LiF determined using the numerical gradient based IVO-CASCI method are in accord with experiment and with other correlated calculations. As an illustration, we may point out that the maximum deviation from the experiment in our estimated normal mode frequency of LiF is 34 cm(-1), whereas for the bond length, the maximum error is just 0.012 A.

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.…”

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

“…The IVO-CASCI orbitals (apart from the occupied SCF orbitals) are simply obtained via an unitary transformation of the HF orbitals with no further refinement required. Several prior applications [53][54][55][56][57][58][59][60][61] demonstrate that these orbitals yield good correlation energies with small reference spaces, a balanced description of ground and excited states, and geometries almost identical to CASSCF geometries (at a fraction of the computational cost).…”

“…[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.…”

“…52,[66][67][68][69][70][71][72][73] One alternative is the IVO-CASCI method of Freed et al [52][53][54][55][56][57][58][59] which has emerged as a computationally more efficient and powerful tool. The IVO-CASCI technique provides an effective alternative to replace the more laborious CASSCF method by eliminating the multiconfigurational orbital optimization step altogether.…”

Studies on m-benzyne and phenol via improved virtual orbital-complete active space configuration interaction (IVO-CASCI) analytical gradient method

“…These features provide a unique niche for the IVO-CASCI method that enables us to treat much larger molecules using the full chemical valence space which, often, is not possible for CASSCF calculations. These features add advantage to IVO-CASCI based dynamical calculations over those of the traditional methods with the MCSCF/ CASSCF scheme [17].…”