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 N2, LiF, and C4H6 (butadiene) molecules, systems that are sufficiently complex to assess the applicability of these methods. The ionic-neutral curve crossing involving the lowest two 1Σ+ 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 N2 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 C4H6 using numerical energy gradients. The ground spectroscopic constants of N2 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˚.