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12

I think the way this question is asked is a little too simplistic. In order to execute a computational project, there is always more than one calculation required. Even if you are happy with the lowest level of theory (say, B3LYP/6-31G*), it does not mean that any package that lists B3LYP in the list of the available features would be useful. Possible ...


11

Multi-reference or single-reference? While it is appreciated that near dissociation there will be a near-degeneracy of orbitals, requiring a multi-reference treatment, fortunately we don't have to worry about this when calculating dissociation energies. For example, the N$_2%$ molecule has profound multi-reference character as you approach the dissociation ...


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 ...


11

Note that the DLPNO method is only implemented in ORCA. There are indeed analogous and similarly efficient and accurate methods, the PNO-LCCSD method in Molpro [doi.org/10.1021/acs.jctc.7b00799] and the LNO-CCSD method in MRCC [doi.org/10.1021/acs.jctc.9b00511]. To my knowledge exact analytical gradients are not implemented for either of them. There is an ...


11

Basis set name versus number of total orbitals I would like to first address a part of the question that appears to be a misconception about the use of a 6-31+G(d,p) basis set, since you wrote: "In my understanding of such basis sets, it is difficult to do this." 6-31+G(d,p) is not a "big" or "small" basis set, unless we're ...


10

Okay, so there are many layers to this question. cc-pVTZ for H is [5s2p1d/3s], which comes out to 3 + 2*3 + 5 = 3+6+5 = 14 basis functions per atom, which are composed of 16 primitives (the contracted s function). Now, while there are 1 and 3 cartesians for the s and p shells, for the d shell you have 6 cartesian functions but only 5 spherical functions. ...


8

One can also pose the opposite question, which may be more interesting: what significant matter modelling methods are implemented in open source software, for which there is no commercial alternative? Anna's answer above had a lot of important considerations, but also this reverse question is important to keep in mind; commercial software is not always free ...


8

BDEs are calculated by the below procedure: Calculate initial energy Perform homolytic bond cleavage and separate fragments Calculate energies of the fragments, add the energies together Calculate BDE by comparing the fragment energies to the initial energy The level of theory and basis set is dependent on how accurate you want the results to be. B3LYP ...


7

As far as I know, analytic gradients for DLPNO-CCSD are not available in ORCA. Analytic first derivatives are available for both closed-shell and high-spin open-shell cases, which could be used for computing other first-order properties. As the first exercise to implement analytic gradients within the DLPNO setup, the DLPNO-MP2 method was considered and the ...


7

MS2 is related to the spin. Specifically, it is the number of unpaired electrons. Your molecule is a singlet, which is why it says 0. ORBSYM is the list of symmetries for each orbital. In this case $\ce{H2}$ is in $D_{\infty h}$ but almost all electronic structure codes do not support non-Abelian groups, so instead we would almost always use $D_{2h}$. The ...


6

One thing the other answers haven't mentioned is the zero point and thermal corrections to the BDE. As mentioned in the Wikipedia Morse potential article, a geometry optimization will take you, by definition to the equilibrium bond length ($r_e$). If you use this as the low energy state, you're calculating $D_e$. The problem is that a quantum harmonic ...


6

The definition of fast and slow is a bit complicated. I just tried an organic molecule with about 100 atoms and ran two jobs: One with def2-TZVP (1647 AO basis sets, 1872 CAOs) and one with cc-pVTZ (1938 AOs, 2205 CAOs). Pure Hartree-Fock single-point energy calculation, both jobs needed 13 SCF iterations. Timings: def2-TZVP 31 minutes, cc-pVTZ 44 minutes ...


6

What systems are you running? Turbomole is designed for basis sets employing segmented contractions (like the Karlsruhe sets are), whereas the Dunning sets are generally contracted. While any code using segmented contractions works also with generally contracted basis sets, it is horrendously slow for heavy atoms since primitive integrals are re-computed ...


5

1RDMs are just simple matrices, which can be stored in either dense or sparse form, possibly combined with triangular storage (the matrix is often symmetric). 1RDMs are passed around in a number of formats, like Gaussian formatted checkpoint and Molden, and can be visualized for density isosurfaces etc. 2RDMs are a bit more problematic, since they can become ...


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 ...


3

"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

In the first sentence of this Introduction to MOLPRO, MOLPRO is described as a "system of programs", not just one program. The first link you provided is for the program called MULTI for MCSCF calculations (which includes multiple types of MCSCF such as CASSCF and RASSCF). The second link you provided is for the program called MRCI. Each program in ...


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{...


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