ABSTRACT
In contrast to valence bond theory (see Topic H2), molecular orbital theory attempts to describe the bonding orbitals in a molecule in their entirety, as opposed to focusing on the creation of each bond individually. Just as the explicit calculation of atomic orbitals is only possible for the hydrogen atom, direct calculation of the molecular orbitals is possible only in the simplest possible molecule, H2+. Unlike atomic wavefunctions, the molecular wavefunction must describe the relative motion of nuclei in addition to the motion of the electrons. Primarily because of their relative masses, the electrons move some 103 times faster than the nucleus, and the Born-Oppenheimer approximation simplifies the calculation of molecular orbitals by assuming that the nuclei are stationary relative to the motion of the electron. This approximation allows the internuclear repulsion terms to be treated completely separately from the electrostatic behavior of the electrons.
