3.3 Which basis set should I use?
For testing or debug, you can use any type of basis sets. You can, of course, use a small basis set like def2-SVP
to test whether automr
works. However, for practical use or to obtain publishable data, please use at least triple-zeta basis set, e.g. cc-pVTZ
, def-TZVP
(.ie. TZVP
in Gaussian), def2-TZVP
or def2-TZVPP
. Results obtained from combinations like MRCISD/def2SVP
, NEVPT2/6-31G(d)
are almost useless. If, unfortunately, you have very limited computational resource, e.g. less than 8 CPU cores, then the cc-pVDZ
basis set is recommended. Additional general recommendations are provided below:
(1) If your system is large, or has complicated electronic structure and you want to see whether automr
works, or you just want to see the workflow of automr
, you can use a small basis set like def2SVP
to go through the computation. After it normally terminates, you can switch to using a larger basis set.
(2) If your system is large (>600 basis functions), you should consider properly simplify your system or use mixed basis sets for different elements/atoms. For example, replace unimportant methyl group(s) by hydrogen atoms, use cc-pVTZ
for important atoms and cc-pVDZ
(or even 6-31G(d)
) for other atoms, etc. Also/Alternatively you can turn on the RI approximation (see Section 4.4.29) in CASSCF and CASSCF-NEVPT2 computations.
(3) Pople-type basis set like 6-311G(d,p)
or 6-31G(d,p)
is less recommended, but can be used for unimportant atoms.
(4) If pseudopotential (PP) is desired, better to use cc-pVTZ-PP
, def2-TZVP
. Be careful with the built-in basis set (with PP) SDD in Gaussian software for elements >= the 4th period. Often there is no d, f polarization functions in the built-in SDD basis set of Gaussian (although the PP part seem pretty good), but the d, f polarization functions are usually important for high-accuracy computations. If you insist on using SDD, you should search in previous papers the data of d, f polarization functions of SDD for your atoms, and add the data to the .gjf file manually.
(5) If your molecule is an anion, e.g. MnO4-, it is strongly recommended to use basis set with diffuse functions, such as aug-cc-pVTZ
, ma-def2-TZVP
. Similar to (1), if your system is large, you can use basis set with diffuse functions for atoms with significant negative charges, and use no diffuse functions for other atoms. If the atoms involving significant negative charges are not so important in your study, you can just use aug-cc-pVDZ
or ma-def2-SVP
.
(6) If all-electron relativistic calculations are desired (DKH2 or X2C), do remember to use basis sets like cc-pVTZ-DK
, x2c-TZVPall
or ANO-RCC
series. All-electron relativistic computations using CASSCF/cc-pVTZ
or cc-pVTZ-PP
with DKH2 is almost non-sense.
(7) Remember that the two basis sets def-TZVP
and def2-TZVP
are used formally in papers, but TZVP
and def2TZVP
are used as the names of basis set in Gaussian syntax.
(8) If you want to compute the NMR shielding constants, basis sets like pcSseg-1
or pcSseg-2
are strongly recommended. Large basis sets like pcSseg-3
, def2-QZVP
or cc-pVQZ
would be better but they are very time-consuming.
(9) If you only want to obtain the radical indices printed by automr
, and accurate electronic energies are not desired, then basis sets such as cc-pVDZ
is sufficient. There is no need to use triple-zeta basis set like def2TZVP
. If your studied molecule is very small (e.g. <10 atoms), then you can still use any triple-zeta basis set.
(10) If you only want to perform energy decomposition analysis, e.g. GKS-EDA, usually def2TZVP
basis set is sufficient. If there is any anion in your studied molecule(s), it is recommended to use diffuse functions only for anion-related atoms. It is not recommended to apply ma-TZVPP for all atoms (which will probably cause basis set linear dependence problems and make SCF in GAMESS converges slowly, even with MOs written in the .inp file). Also, it is recommended to use the implicit solvent model PCM rather than SMD, since PCM is frequently used in original papers of GKS-EDA.
(11) Some special basis sets can be used in the .gjf file of MOKIT, although these basis sets are not included in the Gaussian program. This is because MOKIT put these basis data in $MOKIT_ROOT/mokit/basis/
and can import them when necessary. For example, the following syntax will immediately cause errors in Gaussian, but are supported in MOKIT
Example 1. DKH2 scalar relativistic Hamiltonian with the ANO-RCC-VDZP basis set
#p CASSCF/ANO-RCC-VDZP
mokit{DKH2,CASSCF_prog=ORCA}
Example 2. CASSCF NMR calculation with the pcSseg-1 basis set
#p CASSCF/pcSseg-1
mokit{NMR}