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.

MS

The total electronic spin, which specifies the spin state of a system. The default spin state is a singlet state for Hartree-Fock calculations and the lowest-energy integer-spin state for configuration interaction calculations.

SYNTAX: MS=x

The total spin value \(x\) must be an integer multiple of \(1/2\). For example, MS=0 corresponds to a singlet state, MS=0.5 corresponds to a doublet state, and MS=1 corresponds to a triplet state. Many standard spin states also have their own keywords as convenient synonyms, such as SINGLET, DOUBLET, and TRIPLET. Negative \(x\) values are also allowed and corresponds to an excess of beta electrons over alpha electrons.

The MS keyword targets a specific set of spin states within an MECI or INDO calculation. MECI calculations are not spin-adapted, therefore all integer or half-integer spin states are calculated simultaneously. INDO calculations are spin-adapted, and untargetted spin states are not calculated.

Non-zero spin multiplicity is directly compatible with UHF calculations unless the specified spin state is not physically accessible. Spin-polarized UHF calculations are suitable for studying potential energy surfaces and performing geometry optimizations.

For open-shell RHF calculations, a compatible open shell must be defined using either the C.I. or OPEN keywords. Electrons are statistically distributed over alpha and beta spins in the open-shell RHF Hamiltonian, and the total energy is corrected by a post-SCF configuration interaction calculation to recover the correct spin state. Spin-polarized RHF calculations are suitable for studying electronic excited states through configuration interaction calculations.

The simplest ways to specify a compatible active space for open-shell RHF calculations are either C.I.=2x or OPEN(2x,2x).

Note

Most quantum chemistry software specifies spin states using spin multiplicity, which has a different offset and scaling than the spin value used in MOPAC. For example, a singlet state has a multiplicity of 1, a doublet state has a multiplicity of 2, and a triplet state has a multiplicity of 3.