PROTO — Define prototype MOs.




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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