# Tag Info

35

OpenMOLCAS I'm surprised that the other two answers were given without yet any mention of OpenMOLCAS yet! There is a recent paper[1] about it for which I happen to be one of the many co-authors. MOLCAS has the quality of a commercial software like MOLPRO, since it was a commercial software that people paid big money for since the 1980s, but recently an open ...

25

ORCA ORCA (forum). It is fully featured, efficient, and free for academics. The input is straightforward: # CASSCF(2,4) example from the manual, # section 8.1.7.2, page 108 ! TZVPP TightSCF %casscf nel 2 norb 4 end * xyz 0 1 Be 0 0 0 * Some resources for CASSCF: General functionality is reported in this tutorial. A tutorial dedicated exclusively to ...

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PySCF Personally I would use pyscf, its not the most efficient or user friendly but it's free.

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NWChem NWChem is a commonly use quantum chemistry software package with CASSCF included. NWChem is free, as in both free coffee and free speech. It is under ECL-2.0. It is written in FORTRAN (in case you want to do some development).

12

BAGEL The code is available through GitHub[1] and has been described in WIRES.[2] The main advantages of BAGEL is that it has better memory and parallelization than MOLPRO. For example, MOLPRO (not Free) is restricted to 32 active orbitals. While not a problem for CASSCF, this will severely affect your ability to do CASPT2 calculations. (they have fixed ...

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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 ...

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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

As you said above, nitrogen has 2 s orbitals and one p orbital. However, one would typically freeze the chemically inactive 1s orbital, leaving you 5 electrons in 4 orbitals per atom, or 10 electrons in 8 orbitals for N$_2$; this is denoted $(10e,8o)$. Going beyond the minimal STO-3G basis, in addition to the occupied molecular orbitals (which are poorly ...

7

Columbus Columbus is an alternative package for CASSCF and other multireference methods. There is a free and open-source distribution. It is a bit old fashioned but still widely used. Newest Reference H. Lischka, R. Shepard, T. Müller, P. G. Szalay, R. M. Pitzer, A. J. A. Aquino, M. M. A. do Nascimento, M. Barbatti, L. T. Belcher, J.-P. Blaudeau, et al., J. ...

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 ...

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Psi4 Psi4 (https://psicode.org/) is another open-source alternative (LGPL). I haven't used it myself but it supports CASSCF.

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"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

GAMESS GAMESS (https://www.msg.chem.iastate.edu/GAMESS/) has an MCSCF module, which can perform CASSCF. For generating configurations it can use determinants, or configuration state functions (CSFs). There are first order, quasi-second order, full newton-raphson and other convergers for the iterations. GAMESS can also perform full second order CI. GAMESS ...

3

DALTON For what I know also the Dalton suite is able to run CASSCF. It's also an open source software. You can find the tutorial to run CASSCF calculation here.

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"However they are listed with parity -1, and: "Expectation values are only nonzero for symmetric operators (parity=1)"." This matches the comment by wzkchem5 which suggests that those expectation values, like $\left\langle \hat{L}_x \right\rangle$ which you correctly found to be listed as -1 in the parity column of the table in the link ...

3

You fixed the occupation within symmetries by the command "occ,6,3,3,2" and asked the program to make CASSCF for the lowest 5 singlet states of symmetry 1 ($A_1$). And indeed, as you see, the first 6, 3, 3, and 2 orbitals in symmetries 1, 2, 3, 4, respectively, have nonzero occupation numbers, which sum up to 6+8=14, the number of electrons of $\ce{... 3 So what happens when the contribution of excited determinants are included in the vibrational and rotational terms? Nothing, except that the Hessian and/or gradient are more difficult to evaluate, for example see coupled-perturbed Hartree-Fock. I think maybe your confusion is arising because the excited determinants are not exactly excited-states. A multi-... 2 DOLO (Do Localized Orbital) CASSCF CASSCF can capture static electron correlation (i.e. orbital degeneracies), and it is very important to select the active space which includes the degenerate orbitals. For example, in a bond breaking event the$\sigma$and$\sigma\$* orbital will become degenerate and should be in the active space. However, anyone familiar ...

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