Note

This website is presently under development. The primary website for MOPAC is still http://openmopac.net until the majority of its contents have been migrated here.

UHF

Requests an unrestricted Hartree-Fock Hamiltonian that allows the two electrons of opposite spin in an electron pair to have different spatial orbitals.

For supported features, this is the default behavior for odd-electron systems (i.e. radicals). A UHF calculation can be requested for even-electron systems, but it should produce the same results as a spin-restricted calculation (RHF).

UHF calculations often contain unphysical spin contamination. In the absence of external magnetic fields, a many-electron wavefunction should be an eigenstate of the z-axis spin operator, \(S_z\), and the total spin operator, \(S^2\). The additional orbital relaxations allowed by an RHF calculation breaks the overall spin symmetry, which enables the wavefunction to lower its total energy by deviating from an eigenstate of \(S^2\).