An ab initio nonorthogonal valence bond program, called XMVB, is described in this article. The XMVB package uses Heitler-London-Slater-Pauling (HLSP) functions as state functions, and calculations can be performed with either all independent state functions for a molecule or preferably a few selected important state functions. Both our proposed paired-permanent-determinant approach and conventional Slater determinant expansion algorithm are implemented for the evaluation of the Hamiltonian and overlap matrix elements among VB functions. XMVB contains the capabilities of valence bond self-consistent field (VBSCF), breathing orbital valence bond (BOVB), and valence bond configuration interaction (VBCI) computations. The VB orbitals, used to construct VB functions, can be defined flexibly in the calculations depending on particular applications and focused problems, and they may be strictly localized, delocalized, or bonded-distorted (semidelocalized). The parallel version of XMVB based on MPI (Message Passing Interface) is also available.

Steric effect dominates: Ab initio valence bond and block‐localized wavefunction methods are used to estimate the contributions of hyperconjugation and steric effects to the ethane rotation barrier. The results show that hyperconjugation stabilizes the staggered conformer by about 4 kJ mol−1 relative to the eclipsed form (see picture) and steric hindrance is the major driving force behind the favoring of the staggered conformation in ethane.

Ozonization of the sidewalls of single-wall carbon nanotubes (SWNT) has been investigated by means of 2-layered ONIOM(B3LYP/6-31G*:AM1) calculations. The theoretical calculations reveal that the 1,3-dipolar cycloaddition (1,3-DC) of ozone (O 3 ) onto the sidewalls of a (5,5) carbon nanotube is site-selective and facile with a small activation barrier of 1.4 kcal/mol. The desorption of ozone from the thus-formed ozonide was found to be favorable over the decomposition process that gives rise to epoxy adduct and O 2 upon thermal activation. This work implies the possibility of functionalizing the sidewalls of SWNTs by means of 1,3dipolar cycloadditions of 1,3-dipolar molecules.

The viability of 1,3-dipolar cycloadditions of a series of 1,3-dipolar molecules (azomethine ylide, ozone, nitrone, nitrile imine, nitrile ylide, nitrile oxide, diazomethane, and methyl azide) onto the sidewalls of carbon nanotubes has been assessed theoretically by means of a two-layered ONIOM approach. The theoretical calculations predict the following: (i) other than the 18-valence-electron azomethine ylide and ozone, the 16-valence-electron nitrile ylide and nitrile imine are the best candidates for experimentalists to try; (ii) upon 1,3-dipolar cycloaddition, a 1,3-diople molecule is di-sigma-bonded to a pair of carbon atoms on the sidewall of SWNT, forming a five-membered ring surface species; (iii) the as-formed 1,3-dipole-SWNT bonding is much weaker than that in the products of the molecular 1,3-DC reactions and can be plausibly broken by heating at elevated temperatures; (iv) the sidewalls of the armchair (n,n) SWNTs (n = 5-10) are subject to the 1,3-DCs of ozone and azomethine ylides; (v) both the 1,3-DC reactivity and retro-1,3-DC reactivity are moderately dependent on the diameters of SWNTs, implying the feasibility of making use of the heterogeneous 1,3-DC chemistry to purify and separate SWNTs diameter-specifically.