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| open-shell_coupled_cluster_theories [2023/05/22 14:46] – [Open-shell coupled cluster theories] hesselmann | open-shell_coupled_cluster_theories [2024/01/08 13:24] (current) – external edit 127.0.0.1 |
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| It should be noted that in open-shell cases the triples energy slightly depends on the treatment of core orbitals. In Molpro pseudo-canonical alpha and beta spin orbitals %%[%%[[http://dx.doi.org/10.1016/S0009-2614(91)85118-G|Chem. Phys. Lett.]] **186**, 130 (1991)] are generated by block-diagonalizing the corresponding Fock matrices in the space of valence orbitals, leaving frozen core orbitals untouched. Some other programs include the frozen core orbitals in the canonicalization and transformation. Because of core-valence mixing this leads to slightly different energies. Neither of the two methods can be regarded as better or more justified — it is just a matter of definition. However, the method in Molpro is more efficient since the subsequent integral transformation involves only valence orbitals and no core orbitals. | It should be noted that in open-shell cases the triples energy slightly depends on the treatment of core orbitals. In Molpro pseudo-canonical alpha and beta spin orbitals %%[%%[[http://dx.doi.org/10.1016/S0009-2614(91)85118-G|Chem. Phys. Lett.]] **186**, 130 (1991)] are generated by block-diagonalizing the corresponding Fock matrices in the space of valence orbitals, leaving frozen core orbitals untouched. Some other programs include the frozen core orbitals in the canonicalization and transformation. Because of core-valence mixing this leads to slightly different energies. Neither of the two methods can be regarded as better or more justified — it is just a matter of definition. However, the method in Molpro is more efficient since the subsequent integral transformation involves only valence orbitals and no core orbitals. |
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| Spin-unrestricted coupled cluster calculations can be done using the input | Spin-unrestricted coupled cluster calculations using UHF orbitals can be done using the input options |
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| | <code> |
| | uhf |
| | uccsd|uccsd(t)|ucisd|ump2|ump3|ump4 |
| | </code> |
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| | The spin-unrestricted coupled cluster code can also handle spin-restricted references, yet, by default the restricted open-shell coupled cluster code is executed when the previous SCF calculation was of RHF type. In order to force the execution of the unrestricted code one can set the option ''UHF_UCCSD=1'' to the ''UCCSD'' command. It should be noted, however, that there is little to choose between the two different codes performance wise for RHF references. |
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| ==== Open-Shell Coupled cluster calculation on CN ==== | ==== Open-Shell Coupled cluster calculation on CN ==== |