XMVB 2.0: A new version of Xiamen valence bond program

Abstract

Xiamen valence bond (XMVB), which is an ab initio nonorthogonal valence bond program, has been progressively developed and refined during the last 25 years. As the release of XMVB 1.0 in 2004, a number of significant enhancements and improvements have been made to the program. As a consequence, a new version, XMVB 2.0, has been released and will be described in this article. In XMVB 2.0, the nonorthogonal orbital‐based reduced density matrix approach for the valence bond (VB) theory is implemented, based on the second quantization scheme for nonorthogonal orbitals. The present article also describes the recently developed algorithms for orbital optimization in the VB self‐consist field (VBSCF) method, in which the internal contraction of wave function is used for computing energy gradients. Moreover, several newly implemented ab initio VB methods, such as VBSCF(CAS), internally contracted VB second‐order perturbation theory (icVBPT2), VB polarizable continuum model, VB effective fragment potential (VBECP), and density‐functional‐based VB, are briefly reviewed in this article. Finally, test calculations of several planar arenes, in which up to 18 active electrons are involved, are performed with XMVB 2.0. © 2014 Wiley Periodicals, Inc.

The origins of the directionality of noncovalent intermolecular interactions#

“…While each HLSP function is expanded into 2 N /2− S Slater determinants (where N and S are the total number of electrons and spin quantum numbers, respectively), the major computational obstacle for the ab initio VB methods comes from the nonorthogonality of orbitals. But we note that ab initio VB theory has been rejuvenated remarkably in the past two decades with a few practical programs including xiamen valence bond (XMVB) available . Combining both the advantages of VB and molecular orbital (MO) theories, the BLW method is the simplest variant of the VB theory; it defines a diabatic state with only one Slater determinant but retains the nonorthgonality among orbitals in different subgroups.…”

The application of cholesky decomposition in valence bond calculation

“…The CD‐VBSCF method was implemented in a modified version of XMVB 2.1 program, in which the RDM‐based LBFGS algorithm is adopted. Similar to the previous work, the implementation of CD‐VBSCF consists of the following steps: Reading initial guess for VB orbitals. Evaluating the basis function based ERIs and decomposing them to get basis function based Cholesky vectors by eq.…”

“…However, valence bond (VB) theory still attracts the attentions of many chemists and is considered as one of modern chemical bonding theories, because of its ability to provide intuitive insights for chemical problems . Since the 1980s, more and more ab initio VB methods and programs have been developed and widely applied to explore the nature of chemical bond …”

Performance of the VBSCF method for pericyclic and π bond shift reactions

“…All VB calculations were performed at the optimized geometries with the Xiamen Valence Bond (XMVB) package, which is a quantum chemistry program designed for ab initio VB calculations. The latest version is XMVB 3.0, where Cholesky decomposition is implemented to prepare the electronic repulsion integrals .…”

“…The second category focuses on the spin coupling patterns in VB structures and uses delocalized AOs to pursue the compactness of wave functions. Spin‐couple valence bond and generalized valence bond methods are two outstanding examples in this category, where the former uses nonorthogonal overlap‐enhanced AOs while the latter imposes strong orthogonality condition on delocalized orbitals. The third category is based on MOs and retraces VB concepts quantitatively by imposing the localization on MOs (thus orbitals are essentially semi‐delocalized).…”