23.6 CPHF for gradients, expectation values and polarizabilities

The accuracy and other parameters of the CPHF calculations necessary to compute gradients and response properties
can be modified using the `CPHF` directive:

`CPHF`,
[`THRMIN=`*thrmin*],
[`THRMAX=`*thrmax*],
[`MAXIT=`*maxit*],
[`SHIFT=`*shift*],
[*SAVE=record*],
[*START=record*],
[`DIIS=`*idiis*],
[`DISM`=idism]

`THRMIN`- CPHF convergence threshold (default 1.d-6).
`THRESH`and`ACCU`are aliases for this. `THRMAX`- initial CPHF convergence threshold in geometry optimizations. Once the geometry is converged to
a certain accuarcy (depending on
`OPTCONV`), the threshold is stepwise reduced to`THRMIN`.

The default is`THRMAX`=min(1.d-6,`THRMIN`*100); Values larger than 1.d-6 are ignored. `MAXIT`- Maxmimum number of iterations (default 50).
`SHIFT`- Level shift for CPHF (default 0.1).
`DIIS`- First macroiteration in which DIIS is used (default 1)
`DISM`- First microiteration in which DIIS is used (default 1; in microiterations the core contribution is frozen).
`SAVE`- Record on which the CPHF solution can be saved for later restarts. The solution is saved automatically in geometry optimizations and frequency calculations.
`START`- Record from which initial guess is read. A starting guess is read automatically in geometry optimizations and frequency calculations.

In density-fitting MP2, only `THRMIN` is active, the rest of the options are handled automatically.

molpro@molpro.net 2019-06-17