CASSCF Calculation¶
CASSCF¶
The majority of calculations conducted with HUMMR will be based on a CASSCF or MCSCF calculation as it is the central module of the program. The following is an example for a CASSCF input file.
| casscf.inp | |
|---|---|
Most keywords in the General and
CASSCF blocks are self-explanatory. For example, the
calculation above will feature 4 active electrons in 4 active orbitals and is
allowed to run over 35 orbital optimization cycles. Note, that in HUMMR, both
the total energy and orbital rotation gradient are required to reach
convergence. The choice of orbital optimization algorithm will be critical in
this regard. In the sample input, the perturbative approach of Kollmar et al.
was chosen as standard approach and will be changed to a combination of Pulay’s
DIIS algorithm and Staemmler's first-order method once the norm of the orbital
gradient has fallen below a threshold (cf. SwitchOrbStepThresh).1,2,3
Other options are given above and we strongly advise to make use of different
combinations when convergence is difficult to achieve.
An important aspect of CASSCF, MCSSCF and related calculations (e.g. SC-NEVPT2)
concerns the generation and handling of two-electron repulsion integrals.
HUMMR utilizes the open-source implementation of Frank Neese's SHARK4
integral generation and digestions system provided by Lible5 to evaluate
two-electron integrals. To reduce the fast-growing computational cost related to
the straightforward calculation of four-center integrals, it is possible to
employ the density fitting or resolution-of-the-identity (RI) approximation.6
It is invoked by Ints RI in the General block of the input file (see
example above). Note, that you have to provide a suitable auxiliary basis set
if the RI approximation is used (see BASIS SETS).
Starting Orbitals¶
At this point it should also be noted that the quality of the starting orbitals
is another critical ingredient of any CASSCF calculation and will greatly
influence its convergence behavior. Hence, it is required to read a set of
molecular orbital coefficients from a previous HUMMR or ORCA6 calculation
as guess. If orbitals from an ORCA calculation should be utilized the name of the
.json file needs to be provided after the keyword OrcaJSONName while in case
of HUMMR orbitals the name of the orbital file needs to be precluded by
OrbGuessName.
If the orbital coefficients that are read in as guess refer to a different
basis set than the one in the current calculation the original basis set needs
to be provided after the InputOrbitals keyword. The program will then project
the orbital coefficients on the current atomic orbital basis. With the
OrbGuessRotation keyword the order of the guess orbitals can be changed.
If desired, the set of orbitals that results from a HUMMR calculation (stored
in the calcname.C0 file) can be read into HUMMR as guess for any subsequent
calculation. In addition, HUMMR produces a file calcname.orca.json that can be
converted to a .gbw file by the orca_2json program.
Implicit Solvation¶
As many chemical reactions occur not in the gas phase but in solution, HUMMR offers the possibility to simulate solvation with an conductor-like polarizable continuum model (C-PCM or COSMO). If you wish to include implicit solvation you simply need to add
to your input file. It will invoke C-PCM with a dielectric constant of 4.81 which corresponds to chloroform. Of course, one may change some technical setting for the implicit solvation model, i.e.