35.7 Choosing the ansatz and the level of approximation

The Ansatz can be chosen using the `ANSATZ` option and/or by options on the command line.

`3A`- Ansatz 3A;
`3*A`- Ansatz 3A with EBC approximation
`3B`- Ansatz 3B
`3*B`- Ansatz 3B with EBC approximation
`3C`- Ansatz 3C
`3*C`- Ansatz 3C with EBC approximation

The ansatz can be further detailed by appending options in parenthesis, e.g.

`ANSATZ=3B(D)`

These options can be one of

`D`- Use diagonal ansatz
`FIX`- Use diagonal ansatz with fixed amplitudes (orbital invariant)
`FIXC`- Use diagonal ansatz with fixed amplitudes and canonical orbitals
`DX`- Use diagonal ansatz and assume X-matrix to be diagonal (only for Ansatz 3A)
`GBC`- Use GBC approximation (only for 3B, default in 3A)
`EBC`- Use EBC approximation (same as *)
`HX1`- Use HX1 approximation (only for 3B).
`HY1`- Use HY1 approximation (only for 3B and 3C).
`HY2`- Use HY2 approximation (only for 3B and 3C).
`HY`- Default hybrid approximation. HX1 and HY2 approximation in 3B, HY2 in 3C.
`NOZ`- Neglect Z terms (only for 3B and 3C).
`NOX`- Neglect X terms (only for 3A and 3B).

Several options separated by commas can be given. For instance

`ANSATZ=3C(FIX,HY1)`

uses the diagonal ansatz 3C(D) with fixed coefficients and the hybrid (HY1) approximation.

Alternatively or in addition, the following options can be given on the command line:

`DIAG=1`- Use diagonal ansatz.
`DIAGX=1`- Use diagonal ansatz and assume X-matrix to be diagonal.
`GBC=1`- Use GBC approximation (only for 3B, default in 3A).
`EBC=1`- Use EBC approximation (same as *).
`HYPRID=n`- Use HYn approximation.
`HYPRIDX=1`- Use HX1 approximation.
`NOZ=1`- Neglect Z terms (only 3B and 3C).
`NOX=1`- Neglect X terms (only 3A and 3B).
`FIX=1`- Use diagonal ansatz with fixed coefficient approximation (orbital invariant).
`FIX=2`- Use diagonal ansatz with fixed coefficient approximation. Evaluate only first order energy expression, not the Hylleraas functional. Very fast but less accurate and reliable!
`FIXCAN=1`- Use diagonal ansatz with fixed coefficient approximation and canonical orbitals. A non-iterative method
is used to evaluate the energy. This is equivalent to
`FIX=1,CANONICAL=1`and is most efficient. `FIXCAN=-1`- As
`FIXCAN=1`, but equations are solved iteratively (test purpose only). `CABS=1`- Use CABS (default). If
`CABS=0`is given, CABS is disabled. However, if`RI_BASIS=OPTRI`, the orbital and OPTRI basis sets are automatically merged, and then exactly the same results as with`CABS=1`are obtained. `ORTHO_CABS=1`- Construct CABS basis from orthogonal MOs and ABS basis rather than AO and RI basis.
`THRABS=`*thrabs*- Threshold for smallest eigenvalue of S in auxiliary ABS (only used with
`ORTHO_CABS=1`; default=`THRCABS`). `THRCABS=`*thrcabs*- Threshold for smallest eigenvalue of S in CABS (default 1.d-8).
`THRCABS_REL=`*thrcabs_rel*- Relative CABS threshold (default 1.d-9). The actual threshold is
`max(thrcabs,eigmax*thrcabs_rel`, where`eigmax`is the largest eigenvalue of the overlap matrix. `PRINT=`*level*- Print parameter.
`PRINT=1`give information about all computed integrals and the iterations. `DEBUG=`*level*- Can be used to obtain extended debug print.
`SOLVE=0`- Use a most efficient pair-specific fully iterative method (default).
`SOLVE=1`- Use simple fully iterative method.
`SOLVE=2`- Use pair specific iterative method (more expensive).
`SOLVE=3`- Use pair specific non iterative method (most expensive, only with canonical orbitals).
`CANONICAL=1`- Use canonical orbitals and full domains.
`DOMSEL=1`- Use full domains and localized orbitals (unless
`CANONICAL=1`is given). `SCALE_TRIP=1`- Scale triples energy as explained in section 35.10.
`SINGLES`- If set to one, include CABS singles correction (default=1)
`CORE_SINGLES`- If set to one, include CABS singles correction for core orbitals (default=0)
`EXTGEN`- For open-shell systems: If 1 (default,recommended), include all occupied valence orbital pairs for in , independent of spin (as described in J. Chem. Phys. 130, 054104 (2009), section II.E). If 0, use only pairs mn where the spins of and , and and are equal.

For instance

`ANSATZ=3C,fix=1,hybrid=1,canonical=1`

implies a canonical 3C calculation with diagonal ansatz 3C, using fixed coefficient and hynrid approximations. The
combination of the options `fix=1` and `canonical=1` implies a non-iterative calculation of the energy
and is recommended. The above is equivalent to all of the following:

`ANSATZ=3C(FIXC,HY1)`
`ANSATZ=3C(D,FIXC,HY1)`
`ANSATZ=3C(D,HY1,FIX),canonical=1`

Note that the HF convergence threshold should be rather strict to obtain
accurate results (use `ACCU,14` in the HF).

Numerous further options are for specialist use only and not described here. See `explicit.registry` for a full list.

molpro@molpro.net 2019-06-17