All Questions
Tagged with multi-reference quantum-chemistry
11 questions
3
votes
0
answers
69
views
Is configuration interaction able to produce degenerate ground states?
I am new to multi-determinant theories. What motivates me in asking this question is the following observation: Consider the Dyson orbital of a $\ce{CO2}$ molecule whose axis is parallel to the $z$ ...
- 261
1
vote
0
answers
30
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How can I understand if my system has degenerate electronic configuration?
How can I understand if my system has degenerate electronic configuration?
I'm running some post-HF methods and in the meanwhile try to understand which is the most accurate method for my system. ...
- 11
9
votes
1
answer
1k
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What is meant when people say a molecule has high multi-reference character?
I have read some other posts explaining what multi-reference and multi-configuration are with regards to the wave-function (such as What exactly is meant by 'multi-configurational' and 'multireference'...
- 243
4
votes
0
answers
158
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How can I save and read the CI vector during CASSCF calculations in Gaussian?
I have searched through the GAUSSIAN 16 manual and have not been able to solve these two related issues:
(1) I want to be able to save the CI vector and load it as an initial guess for a different ...
- 1,026
5
votes
0
answers
447
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What is the theoretical justification for state averaging in SA-CASSCF?
According to the molpro manual, section 7.3:
In order to compute excited states it is usually best to optimize the
energy average for all states under consideration. This avoids
root-flipping ...
- 1,026
24
votes
2
answers
8k
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What exactly is meant by 'multi-configurational' and 'multireference'?
Some topics here have touched on this before (see 1, 2, 3), but I haven't found a clear definition yet.
I would like to know what exact property of the wave function these terms refer to. It would ...
- 1,459
9
votes
2
answers
2k
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The difference between CASSCF and MCSCF methods
I'm not sure the exact difference between the MCSCF and CASSCF methods. I know that the CASSCF method proceeds by optimizing both the CSFs and MOs (I mean the coefficients), but I don't know if this ...
- 183
12
votes
2
answers
2k
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How does optimizing the molecular orbital coefficients in CASSCF improve its multi-reference capabilties?
The complete active space self-consistent field (CASSCF) method is a special kind of multi-configurational SCF procedure, attempting to combine Hartree-Fock (HF) and configuration interaction (CI). In ...
- 4,773
6
votes
1
answer
857
views
How can I choose the right symmetry in CAS calculation using MOLCAS?
With $C_{2\mathrm{v}}$ symmetry and a closed shell configuration, how can I choose the occupation for each irreducible representation in the input file?
- 109
10
votes
1
answer
2k
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Why is no dynamical correlation recovered in the CASSCF method?
Configuration interaction is the only certain way to get a calculation to converge toward the exact solution of the many-body Schrödinger equation (within the approximations used for the framework of ...
- 4,773
7
votes
1
answer
2k
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Why is the CASSCF method multi-configurational, while the CI method is not?
The CASSCF method is perhaps the most commonly used theoretical method for studying difficult chemical systems exhibiting multi-reference character or non-dynamical/static/strong correlation. CASSCF ...
- 4,773