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VB2000 is an ab initio electronic structure package for performing modern Valence Bond (VB) calculations based on a highly efficient VB algorithm - the so called Algebrant Algorithm. It has also a general implementation of the Group Function (GF) Theory, in which a large molecule is described in terms of its constituent parts of physically identifiable "electron groups". The combination of the efficient VB algorithm and the general implementation GF theory in VB2000 makes it a useful tool for the investigation of the nature of chemical systems.

Features of VB2000:

Hartree-Fock Molecular Orbital Theory

Hartree-Fock molecular orbitals are generated before any type of VB calculations. The HF orbitals are then used for generating an initial approximation to group functions and their corresponding initial one-electron densities. Pipek-Mezey Localization Method The Pipek-Mezey (PM) method for localization of molecular orbitals (MOs) has been implemented.

Modern ab initio VB Method

VB2000 can be used for very general VB calculations:

Multi-Structure VB (VB)
Spin-Coupled VB (SCVB)
Complete Active Space VB (CASVB)
VB with Localized Electron Pairs (GVB)
VB Configuration Interaction (VBCI)
VB calcualtion with dynamic VB orbitals (BOVB type calculation).

Group Function (GF) Theory

The general form of the GF theory has been fully implemented in VB2000. In general, a molecular wave function is expressed as an antisymmetrized product of individual group functions.The CASSCF and GVB type of wave functions are the special cases in the GF framework. Energy Profile of Chemical Reactions

VB2000 provides a functionality for the automatic calculation of the energy profile of a chemical reaction along the reaction path guided by a few intermediate structures (such as reactant, transition state, and product etc.).

Localization of VB orbitals

VB2000 provides three methods for obtaining more localized VB orbitals during orbital optimization.

Maximal localization of VB orbitals for CASVB wave functions.
Strict localization of VB orbital optimization.
Enhanced localization of VB orbital optimization.

Structure Weight Analysis

Three methods for structure weight analysis have been implemented, i.e. the Mulliken type (i.e. the Chirgwin-Coulson type), the Lowdin type, and the Hiberty type.

Spin Density of VB Wave Functions

Spin density calculation has been introduced in VB2000 version 1.8(R2).

Visualization of VB Orbitals

Visualization files can be created so that VB oribtals can be visualized with third party programs, such as VMD and Molekel.