# Tag Info

11

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 necessary since the 2019 manual seems to answer your question. MOLPRO has 3 different programs for calculating gradients: CADPAC, ALASKA/SEWARD, and AIC. The ...

10

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 for any mixing of the Kohn-Sham states, in fact it would lead to an incorrect result. In practice, the exchange-correlation functional is approximated and so ...

10

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 defined in two non-overlapping sets of orbitals: a single determinant of occupied inactive orbitals, and a general correlated wave function describing the ...

8

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 to right. Let's take only the ground state determinant as reference and do CIS within a space of 4 spatial orbitals (yielding 8 spin orbitals in total) and ...

8

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-AQCC, and even MR-CC will start with a multi-configurational SCF (MCSCF) calcualtion. There's many types of these, and many can be found here: What are the types ...

8

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 calculations, can take a bit more mental energy if you want to make sure your active space is appropriate, but on the other hand, the result could be much more ...

7

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 additional configurations from all the possible rearrangements (hence, complete active space) of the electrons among those orbitals. As the name suggests, a RAS ...

6

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; alternatively, you can form UHF, MP2, or CISD etc natural orbitals and pick the active space based on them. I even believe that you can input your own integrals so ...

6

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", since swapping two orbitals is equivalent to applying a unitary rotation of 90 degrees. The "90"s can actually be omitted, but I'm writing them out to illustrate that you can plug ...

6

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 configurations $\Phi_1$ and $\Phi_2$ (herein we have only two active orbitals, $\varphi_1$ and $\varphi_2$), and also (2) the coefficients $C_1$ and $C_2$, subject to (1) ...

4

"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 states close to the ground state, the approximation you said Gaussian uses, where excited state contributions are neglected, seems not to be such a great idea ...

4

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 the ANO-RCC basis set and got a different occupancy pattern compared to you: Final alpha occupancy: 14 6 6 2 Final beta occupancy: 14 5 5 2 I would recommend to find the ...

4

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 smaller than 1 (this is not a complete list, but it contains molecules that I've either worked on recently or ones that others in the multi-reference community ...

3

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 memory would eliminate the error. The error remained if I increased the memory, explicitly included an RWF file, and if removed the polarization functions (6-31g ...

3

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, for example see coupled-perturbed Hartree-Fock. I think maybe your confusion is arising because the excited determinants are not exactly excited-states. A multi-...

2

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 orbitals. For example, in a bond breaking event the $\sigma$ and $\sigma$* orbital will become degenerate and should be in the active space. However, anyone familiar ...

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