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Abstract The global minima of urea and thiourea were characterized along with other low‐lying stationary points. Each structure was optimized with the CCSD(T) method and triple‐ζcorrelation consistent basis sets followed by harmonic vibrational frequency computations. Relative energies evaluated near the complete basis set limit with both canonical and explicitly correlated CCSD(T) techniques reveal several subtle but important details about both systems. These computations resolve a discrepancy by demonstrating that the electronic energy of the C2vsecond‐order saddle point of urea lies at least 1.5 kcal mol−1above the C2global minimum regardless of whether the structures were optimized with MP2, CCSD, or CCSD(T). Additionally, urea effectively has one minimum instead of two because the electronic barrier for inversion at one amino group in the Cslocal minimum vanishes at the CCSD(T) CBS limit. Characterization of both systems with the same ab initio methods and large basis sets conclusively establishes that the electronic barriers to inversion at one or both NH2groups in thiourea are appreciably smaller than in urea. CCSDT(Q)/cc‐pVTZ computations show higher‐order electron correlation effects have little impact on the relative energies and are consistently offset by core correlation effects of opposite sign and comparable magnitude.
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This content will become publicly available on August 27, 2026
How ‘ de facto variational’ are fully iterative, approximate iterative, and quasiperturbative coupled cluster methods near equilibrium geometries?
Abstract While limited coupled cluster theory isformallynonvariational, it is not broadly appreciated whether this is a major issuein practice. We carried out a detailed comparison withde factofull CI energies for a relatively large and diverse set of molecules near equilibrium geometries. Fully iterative limited CC methods such as CCSDT, CCSDTQ, and CCSDTQ5 do represent practical upper bounds to the FCI energy, as do CCSDT-3, CCSDTQ-3, and CCSD(cT). While quasiperturbative approaches such as CCSD(T) and especially CCSDT(Q) may significantly over-correlate molecules if there is significant static correlation, this is much less of an issue with Lambda approaches such as CCSDT(Q)Λ.
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- Award ID(s):
- 2430408
- PAR ID:
- 10649469
- Publisher / Repository:
- De Gruyter
- Date Published:
- Journal Name:
- Pure and Applied Chemistry
- Volume:
- 97
- Issue:
- 10
- ISSN:
- 0033-4545
- Page Range / eLocation ID:
- 1383 to 1392
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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