11
votes
Accepted
Can MOLPRO (2012) calculate SA-CASSCF gradients with cc-pVDZ?
The MOLPRO 2012.1 manual is no longer online, although there is an archive of at least some of the pages and you can try your luck for finding the pages you want on the archive, but it might not be ...
11
votes
Is there any relation between static correlation and finite-temperature smearing?
This is a nice thought, but the two are not actually related. In principle, static correlation is completely accounted for in the exchange-correlation functional of Kohn-Sham DFT and there is no need ...
- 8,277
10
votes
Accepted
What are the types of MCSCF?
LASSCF: Localized Active Space SCF
An approximation to or generalization of (depending on how you look at it) CASSCF. In CASSCF, the wave function consists of an antisymmetrized product of two factors ...
- 116
9
votes
Accepted
What are the references in multi-reference methods?
I hope this basic example helps you to see the difference. I'll denote configurations by simple tuples where each number indicates the occupation of a spatial orbital, with increasing energy from left ...
- 951
8
votes
What are the references in multi-reference methods?
Rather than starting with a single-determinant (single-reference) such as the Hartree-Fock determinant which is obtained by an HF-SCF calculation, multi-reference methods such as MRCI, MR-ACPF, MR-...
8
votes
Accepted
What is a good practical way to determine whether or not I need to use a multiconfigurational method?
which way is more correct and methodologically appropriate?
Nothing is more "correct" or less correct, but there's certainly advantages and disadvantages. MCSCF calculations such as CASSCF ...
7
votes
What are the types of MCSCF?
RASSCF: Restricted Active Space SCF
In a Complete Active Space (CAS) calculation, one chooses a set of occupied/virtual orbitals (the active space) from an initial Slater determinant and forms ...
- 15.7k
7
votes
Accepted
Modify ORCA orbitals for CASSCF
You can use the "rotate" keyword in the SCF block, like
%scf rotate {30,27,90} (38,41,90} {39,48,90} end end
The "90"s mean "90 degrees&...
- 9,981
6
votes
Accepted
Does PySCF support CAS-SCF calculation?
Yes, it does, and it is even quite efficient in it. Choosing the active space is also quite simple to do with the AVAS method if you know which atoms' valence orbitals you want to make active; ...
- 20k
6
votes
Accepted
Multi-configurational self-consistent field coefficients
In a MCSCF approach, the coefficients $C_I$ are calculated by minimizing the energy $\langle \Psi_{\mathrm{MC}}|H|\Psi_{\mathrm{MC}} \rangle$ with respect to (1) the MOs contributing to the ...
- 9,981
6
votes
Accepted
How to detect an "open shell" molecule?
When you're doing an electronic structure calculation, the input will include the following things:
Geometry
Basis set
Number of electrons
Spin multiplicity
If the spin multiplicity is not 1 (i.e. ...
4
votes
Accepted
Excited determinants, electronic partition function and thermochemistry calculation
"This works for organic molecules, but what happens when the excited states are closer in energy to the ground state, for example in open-shell molecules or in atoms?"
If there's excited ...
4
votes
Accepted
How to determine occupied and closed orbitals for a Molpro CASSCF calculation?
The occupied orbital pattern of 13 7 6 2 that you got from your Hartree-Fock calculation is not unique. For example, I've just run an RHF calculation on PdO with ...
4
votes
Is there any relation between static correlation and finite-temperature smearing?
You may like to read this 2012 JCP paper to know TAO-DFT:
https://aip.scitation.org/doi/10.1063/1.3703894
Even with the exact exchange-correlation energy functional in KS-DFT, one still needs to ...
- 83
4
votes
Are multiplet free atoms multireference?
Open shell atoms are multireference pretty much by definition. Take fluorine, for example, which has a $1s^2 2s^2 2p^5$ electron configuration and a $^2P$ ground state. We know that the electronic ...
- 20k
4
votes
Which molecules have multi-reference character?
The following small- to medium-sized molecular have multi-reference character, meaning that your CISD calculation will likely show the reference wavefunction to have a weight that is quite a fair bit ...
3
votes
How to choose the number of roots in a CASSCF calculation?
The term "roots" refers to the number of "electronic states". If you are only interested in the ground state, then you are seeking 1 root. If you want the ground state and the ...
3
votes
Frequency error in Gaussian CASSCF(10,10)
This is not quite an answer yet, but I wanted to collect some things I found.
Since it looked like this was some kind of overflow error, I tried to see if shrinking the calculation or increasing the ...
- 15.7k
3
votes
Accepted
CASSCF and thermochemistry—effect of excited determinants on rotational and vibrational modes
So what happens when the contribution of excited determinants are included in the vibrational and rotational terms?
Nothing, except that the Hessian and/or gradient are more difficult to evaluate, ...
- 8,015
3
votes
What are the ways to add dynamic correlation to an MCSCF wavefunction?
I can comment on externally corrected coupled-cluster (ec-CC) methods, as I have some direct experience in this topic.
Externally Corrected CC
Externally corrected CC methods seek to improve the ...
2
votes
What are the types of MCSCF?
DOLO (Do Localized Orbital) CASSCF
CASSCF can capture static electron correlation (i.e. orbital degeneracies), and it is very important to select the active space which includes the degenerate ...
- 8,015
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