I am the beginner for gaussian-16 package.
If anyone have the idea about how the frozen-cores are taken into account when we have a molecule with two or more different elements in the Gaussian input file, please give some suggestions.
Frozen-core approximation is mostly used in computationally demanding calculations such as CI-based approaches (CISD, CCSD(T)...). The core orbitals are lower in energy and quantum-chemically not that interesting because during chemical processes these orbitals close to the core (inner orbitals) are not affected much. The outer orbitals are higher in energy and mostly participate in a chemical bonding e.g. in molecules which justifies the usage of this "frozen-core" approximation. One could also explain this by making use of the "window technique" for substituting determinants of a CI calculation. For instance, in a "frozen core" approximation the substitution of electrons in the "inner shell orbitals" are forbidden.
A minimal example how a Gaussian input could look like is given below: #p ccsd=fc/aug-cc-pvdz geom=connectivity ... Hope this helps!
Thank you Okan. I will try with this. But if this is the case then the question comes : how do I know that this "fc" keyword will freeze upto which orbital for different different atoms? This may be a silly question but if u give some idea that will be helpful for me..
Thank you.
The number of core orbitals of each element is already known according to chemistry knowledge, since lowest lying MOs are mainly composed of core atomic orbitals, the number of lowest MOs to be frozen can be simply obtained by counting all core orbitals of atoms presented in current system.
By default, in Gaussian, all atomic orbitals except for the outermost valence ones are regarded as core atomic orbitals. This typically works reasonably. However, in rare case this treatment may cause unsatisfactory result; for example, if you optimize Li2, you will find the discrepancy between CCSD(T,full) and CCSD(T) is never small. Because Li only has one valence electron, electron correlation due to its 1s electrons also have nonnegligible contribution to chemically interesting quantites.
In Gaussian there are a few different ways of determining how many orbitals will be frozen, see http://gaussian.com/frozencore/. For example, with "FreezeInnerNobleGasCore" keyword, MOs composed of both outermost valence and sub-valence atomic orbitals will not be frozen.
If you are still confusing, you can try to calculate some elements and simple molecules, the number of frozen MOs can be found after "NFC=" label in output file.
You are welcome Megha! I would also start with an example in order to get a feeling for this "window technique" I was talking about. The best is that you start e.g. with a simple example such as C4H4 molecule (consisting of 28 electrons and 14 occupied orbitals for a closed shell calculation) and transfer it to your system. For less CPU time, I would recommend to calculate the quantities using MP2. Please follow the link below:
http://thiele.ruc.dk/~spanget/help/g09/k_fc.htm
By the way, the strong contributon in the case of Li2 by the 1s electrons, I would relate to the strong contraction of the 2s orbitals so that they feel the complete nuclear charge because of less shielding by (missing of) orbitals of different symmetry. At least this how I would explain this enhanced effect of the 1s eIectrons on "chemically/physically" interesting quantities and I hope that I am not mistaken...
Thank you Okan and Tian! I went through the window and frozen core keywords as suggested by you. But again I am facing some problem when I am performing a DFT calculation, for example: consider the case of opt+freq calculation for AuH. I performed the calculation with LanL2DZ basis for Au and 6-31G for H atom. But some error is coming. Because LanL2Dz basis is itself an ECP type basis set and "gen" keyword is performs all electron calculation. May be because of that such error is coming. But I don't know how to resolve this problem. I am attaching the input file and the corresponding error which I am getting. I went through the example files given in g16 but didn't find anything related to this problem. So if you are getting the thing please give some suggestions.
Input:
#P pbepbe/gen opt freq
AuH with spin 0
0 1
Au 0.0 0.0 0.00
H 0.0 0.0 1.50
Au 0
lanl2dz
****
H 0
6-31G
****
Error:
There are 14 symmetry adapted cartesian basis functions of A1 symmetry.
There are 2 symmetry adapted cartesian basis functions of A2 symmetry.
There are 5 symmetry adapted cartesian basis functions of B1 symmetry.
There are 5 symmetry adapted cartesian basis functions of B2 symmetry.
There are 12 symmetry adapted basis functions of A1 symmetry.
There are 2 symmetry adapted basis functions of A2 symmetry.
There are 5 symmetry adapted basis functions of B1 symmetry.
There are 5 symmetry adapted basis functions of B2 symmetry.
There are 40 occupied orbitals but only 24 basis functions!
Error termination via Lnk1e in /home2/gaussian/gaussian-16/g16/l301.exe at Mon Dec 18 12:30:58 2017.
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