Note

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MICROS

Restricts the active space of configuration interaction calculations to a list of microstates specified in the input file after the geometry and symmetry data.

SYNTAX: MICROS=n

The microstate list contains \(n\) entries.

The list is preceeded by a line containing MICROS and is separated from the geometry and symmetry data by a blank line. The remaining \(n\) lines each contain the specification of one microstate as a string consisting of 0’s and 1’s. For \(m\) active orbitals (specified by C.I.=m, C.I.=(m,*), or OPEN(*,m)), each microstate defines the occupation of alpha and beta orbitals with a list of \(2m\) occupations, where 0 represents unoccupied and 1 represents occupied. The ordering of active orbitals is consistent with MOPAC’s output, with all active alpha orbitals ordered by increasing energy followed by all active beta orbitals ordered by increasing energy.

A simple example of the MICROS keyword is

C.I.=4 MS=1 MICROS=4
a polarized carbon atom with a full alpha 2p shell

C 0.0 0.0 0.0

MICROS
01111000
01110100
01110010
01110001

All microstates must contain the same total number of electrons for MOPAC to produce sensible results. They should also contain the same numbers of alpha and beta electrons, but this isn’t strictly necessary. Differing numbers of alpha and beta electrons correspond to different spin sectors of configuration space that are not coupled by the many-electron Hamiltonian.

For a configuration interaction calculation to have well-defined spin states, the list of microstates must be closed under the application of the spin operator, \(S^2\). For example, the microstate 0110 should be joined by 1001 so that the two states can be combined into well-defined singlet and triplet states.

For a configuration interaction calculation to have well-defined symmetry states, the list of microstates must be closed under the application of symmetry operators. For example, the microstate 100000 of an atomic p-shell active space should be joined by 010000 and 001000.