NBOvis

In quantum chemistry, a natural bond orbital or NBO is a calculated bonding orbital with maximum electron density. The NBOs are one of a sequence of natural localized orbital sets that include "natural atomic orbitals" (NAO), "natural hybrid orbitals" (NHO), "natural bonding orbitals" (NBO) and "natural (semi-)localized molecular orbitals" (NLMO). These natural localized sets are intermediate between basis atomic orbitals (AO) and molecular orbitals (MO). Natural (localized) orbitals are used to calculate the distribution of electron density in atoms and in bonds between atoms. They have the "maximum-occupancy character" in localized 1-, 2- and 3-center regions of the molecule. Natural bond orbitals (NBOs) include the highest possible percentage of the electron density, providing the most accurate possible “natural Lewis structure” of ψ, which is close to the chemical approach. There was a high demand in our group on displaying these natural bond orbitals, so a JMol based simple user interface was developed for this purpose.
Alpha version is available upon request from jimre(at)chem.elte.hu.
http://www.chem.elte.hu/departments/jimre/NBOvis/