An Exchange-Based Diagnostic for Static Correlation

We propose here a DFT-based diagnostic for static correlation %TAEX[TPSS@HF - HF] which effectively measures how different the DFT and HF exchange energies for a given HF density are. This and %TAEcorr[TPSS] are two cost-effective a priori estimates for the adequacy of the importance of static correlation. %TAEX[TPSS@HF - HF] contains nearly the same information as the earlier A diagnostic, but may be more intuitive to understand. Principal component and variable clustering analysis of a large number of static correlation diagnostics reveals much of the variation is explained by just two components, and almost all of it by four; these are blocked by four variable clusters (single excitations; correlation entropy; double excitations; pragmatic energetics).Comment: 5 pages, AIP Conference Proceedings, in press (ICCMSE-2021

The S66 Noncovalent Interaction Benchmark Re-examined: Composite Localized Coupled Cluster Approaches

The S66 non-covalent interactions are studied through localized coupled-cluster methods and general LNO-CCSD(T)-based composite schemes. Very small RMS deviations (\leq 0.05 kcal/mol) for the low-cost composite approaches from the SILVER reference interaction energies of S66 indicate that we can safely avoid carrying out the largest basis set calculations with veryVeryTight thresholds, and apply instead additivity corrections in smaller basis sets. Interestingly, the counterpoise corrections do not have an appreciable effect on the composite schemes. These findings may prove useful for intermolecular and intramolecular NCIs of larger systems.Comment: 6 pages, AIP Conference Proceedings, in press (ICCMSE-2021

S66x8 Noncovalent Interactions Revisited: New Benchmark and Performance of Composite Localized Coupled-Cluster Methods

The S66x8 noncovalent interactions benchmark has been re-evaluated at the "sterling silver" level, using explicitly correlated MP2-F12 near the complete basis set limit, CCSD(F12*)/aug-cc-pVTZ-F12, and a (T) correction from conventional CCSD(T)/sano-V{D,T}Z+ calculations. The revised reference value disagrees by 0.1 kcal/mol RMS with the original Hobza benchmark and its revision by Brauer et al, but by only 0.04 kcal/mol variety from the "bronze" level data in Kesharwani et al., Aust. J. Chem. 71, 238-248 (2018). We then used these to assess the performance of localized-orbital coupled cluster approaches with and without counterpoise corrections, such as PNO-LCCSD(T) as implemented in MOLPRO, DLPNO-CCSD (T1) as implemented in ORCA, and LNO-CCSD(T) as implemented in MRCC, for their respective "Normal", "Tight", and "very Tight" settings. We also considered composite approaches combining different basis sets and cutoffs. Furthermore, in order to isolate basis set convergence from domain truncation error, for the aug-cc-pVTZ basis set we compared PNO, DLPNO, and LNO approaches with canonical CCSD(T). We conclude that LNO-CCSD(T) with veryTight criteria performs very well for "raw" (CP-uncorrected), but struggles to reproduce counterpoise-corrected numbers even for veryVeryTight criteria: this means that accurate results can be obtained using either extrapolation from basis sets large enough to quench basis set superposition error (BSSE) such as aug-cc-pV{Q,5}Z, or using a composite scheme such as Tight{T,Q}+1.11[vvTight(T) - Tight(T)]. In contrast, PNO-LCCSD(T) works best with counterpoise, while performance with and without counterpoise is comparable for DLPNO-CCSD(T1). Among more economical methods, the highest accuracies are seen for dRPA75-D3BJ, {\omega}B97M-V, {\omega}B97M(2), revDSD-PBEP86-D4, and DFT(SAPT) with a TDEXX or ATDEXX kernel.Comment: Final published version with CC licens

Post-CCSD(T) corrections to bond distances and vibrational frequencies: the power of Λ\Lambda

The importance of post-CCSD(T) corrections as high as CCSDTQ56 for ground-state spectroscopic constants ($D_e$, $\omega_e$, $\omega_ex_e$, and $\alpha_e$) has been surveyed for a sample of two dozen mostly heavy-atom diatomics spanning a broad range of static correlation strength. While CCSD(T) is known to be an unusually felicitous `Pauling point' between accuracy and computational cost, performance leaves something to be desired for molecules with strong static correlation. We find CCSDT(Q)$_\Lambda$ to be the next `sweet spot' up, of comparable or superior quality to the much more expensive CCSDTQ. A similar comparison applies to CCSDTQ(5)$_\Lambda$ vs. CCSDTQ5, while CCSDTQ5(6)$_\Lambda$ is essentially indistinguishable from CCSDTQ56. A composite of CCSD(T)-X2C/ACV5Z-X2C with [CCSDT(Q)$_\Lambda$ -- CCSD(T)]/cc-pVTZ or even cc-pVDZ basis sets appears highly effective for computational vibrational spectroscopy. Unlike CCSDT(Q) which breaks down for the ozone vibrational frequencies, CCSDT(Q)$_\Lambda$ handles them gracefully.Comment: Mol. Phys., in press [Timothy J. Lee memorial issue

Computational study of structural, vibrational and electronic properties of the highly symmetric molecules M4S6 (M = P, As, Sb, Bi)

A systematic computational investigation of the structural, electronic and vibrational properties of the group 15 sulfides M4S6 at Td symmetry was carried out. The performance of DFT and MP2 theoretical methods was assessed compared to the high-level CCSD method. The M–S bond is based on the association between p valence orbitals of M and the 3p of sulfur according to the natural population analysis. Both polarizability and polarizability volume of the cage molecules increase as the size of the atoms increases from P to Bi. A structural ‘relaxation’ ongoing from phosphorus to bismuth showed an increase of ionic character and it might explain the chemical instability of the heavier cage compounds. For the P4S6 molecule, the functionals wB97XD and CAMB3LYP yielded excellent structural data, while for the heavier molecules As4S6, Sb4S6 and Bi4S6, the M06 and M06L functionals showed high accuracy. We validated eight functionals BP86, M06L, B3LYP, M06, Μ06-2Χ CAMB3LYP, wB97XD, B2PLYP which span from conventional GGA functionals to long-range corrected hybrid ones, and MP2, CCSD ab initio methods. Experimentally, these molecules could be useful in the structural investigation of the isolated gas phase species, besides solving complex structures of liquid, crystalline or amorphous phases. © 2019 Elsevier B.V

Vinblastine: cholesterol interactions in lipid bilayers

The anti-mitotic character of vinblastine (VLBS) may relate to its lipophilic nature leading to its hydrophobic interaction with proteins or to its perturbation in lipid bilayers that decreases the lipid fluidity. For these VLBS actions, our work focused on the effects of vinblastine in lipid bilayers. Our aim is to highlight the effects of this highly lipophilic vinca molecule in lipid bilayers in an attempt to use this information in the future for its delivery in liposomal systems avoiding some of its detrimental side effects. Thus, a combination of Differential Scanning Calorimetry (DSC), Raman spectroscopy, X-ray diffraction and Molecular Dynamics (MD) has been applied to study the interactions of antineoplastic VLBS in lipid bilayers. VLBS appears to be distributed to the hydrophilic and hydrophobic segments of lipid bilayers. When cholesterol (CHL) is present in the membrane, VLBS associates not only with the hydrophilic and hydrophobic segments, but also with CHL. This results in the interference of their dynamic effects and causes an antagonistic influence. Thus, in a real biological environment, the interaction between VLBS molecules and cholesterol-rich domains may be disfavored, and a VLBS–CHL separation may occur. Additionally, in our simulations VLBS molecules that were initially placed in the water layer spontaneously entered the lipid bilayer and incorporated into the outer part of the membrane. Finally, we predict that the presence of VLBS favors interdigitation of membrane's alkyl chains. © 2019 Elsevier Inc