PROTO — Define prototype MOs.

PROTO=`n`

During the course of some geometry optimizations, the canonical order of the MOs may
evolve. MOs may cross, possibly resulting in abrupt changes in energy due to discontinuities
in the CI active space. Such changes are automatically prevented in the course of geometry
optimization involving a gradient computation by the numerical method (keyword
DERINU) or a Hessian matrix evaluation, but
not in the course of a normal geometry optimization. When continuity of the CI active MOs is
required, a set of “prototype” CI-active MOs can be defined for use throughout the calculation
using this keyword. After each SCF calculation, the prototype CI-active MOs are used to select
a subset of the new set of CI-active MOs which are most similar to them. The criterion of
similarity is the sum of the squares of the overlap between the prototype CI-active MOs and
the new CI-active MOs. If specified, the value for n may be either 1 or 2. If n = 1 (or simply
PROTO), the first CI-active MOs will be prototypes for the entire calculation, without update.
If n = 2, the calculation is begun as before, but the newly selected CI-active MOs become
prototypes for the subsequent CI calculation, i.e., the prototype CI-active MOs evolve with
the geometry. For geometry optimizations, PROTO=2 may be useful, but for
CHAIN calculations, reaction coordinate
calculations or grid calculations, PROTO=1 should be used since the corresponding geometries
should usually be computionally independent. See Chapter 11, *Configuration Interaction* for
a more complete discussion of AMPAC’s CI capability.

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