MS=n can be used in defining the spin-state of a system. Small values of spin can also be defined using explicit keywords, such as SINGLET, DOUBLET, TRIPLET, QUARTET, QUINTET, SEXTET, SEPTET, OCTET, and NONET. Explicit spin-states can be defined using MS=n, for example MS=2.5 is equivalent to SEXTET. Higher values of MS=n are allowed, e.g. MS=20.
RHF: Use RHF when electronic phenomena, such as excited state energies, transition dipoles, etc. are needed. As with the explicit spin-state keywords, an open-shell must be defined. See C.I.=n, C.I.=(n,m), and OPEN(n1,n2).
UHF: Use UHF for chemistry, including geometry optimization, transition states, reaction paths, etc. If the spin-state is not possible, an error message will be printed.