Numerical Gradients *CCGR

This section is used in the calculation of numerical derivatives of the CC energy. Since it is numerical it can be used for all models and both ground and excited states.

For excited states there is the problem of specifying which excited state is to be studied - and keeping track of this. One can specify the excited state by symmetry and number, using the keywords below. This works fine for gradients (though ordering may change in course of the optimization, but excited state optimization will inevitably be less black box than ground state optimizations), but can fail when there is symmetry lowering in the calculation of numerical hessians. For this purpose one can give the excitation energy for the appropriate state at a lower level (for example CCS) and from that find symmetry and number used in the real higher level calculation. It is implemented by the keywords in the CCEXCI section.

.NUMGD

Specify that the calculation of gradients is to be done numerically. All that is required for coupled cluster (in addition to appropriate minimization, keywords see Chapter 7).

.XSTNUM

READ (LUCMD,*) IXSTAT
The number of the excited state for which the gradient is to be calculated (counted in terms of increasing energy).

.XSTSYM

READ (LUCMD,*) IXSTSY
Symmetry for excited state for which gradient is to be calculated.