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.

ROOT

Target an excited state in a configuration interaction calculation. This keyword can be combined with MS to target a state based on spin, energy, and optionally symmetry.

SYNTAX: ROOT=n

Target the \(n\), which corresponds to the lowest energy state.

When combined with the MS keyword, only the specified spin states are considered in this selection process.

For example, ROOT=2 targets the first excited state. When combined with MS=1, it targets the triplet state with the second lowest energy among all triplet states.

SYNTAX: ROOT=symbol

Target an eigenstate corresponding to a combined quantum number and symmetry label, symbol. symbol is a string concatenating quantum numbers (Q.N.) and symmetry labels as printed by MOPAC.

When multiple spin states have the same quantum number and symmetry label, the lowest-energy spin state will be targeted. Again, the MS keyword can be used to target a specific spin state.

For example, ROOT=2T2g targets the second-lowest energy state having T2g symmetry. When combined with MS=1, it targets the second-lowest energy that is both a triplet state and has T2g symmetry.

Note

For symmetries that induce energy degeneracies, the subspace of energy-degenerate eigenstates are counted and printed by MOPAC as a single state. State labels are skipped to account for and denote this degeneracy. Thus, ROOT=n may not target the state labeled by n if there are any degenerate subspaces of lower energy.

When a degenerate subspace of states is targeted, the reference state becomes a statistical superposition over the degenerate states so that geometry optimization is able to preserve symmetries. To target a specific state in a degenerate subspace so as to break symmetry in the geometry optimization (i.e. Jahn-Teller distortion), the NOSYM keyword should be used to suppress the detection and use of point-group symmetry.