Exploring the Accuracy Limits of Local Pair Natural Orbital Coupled-Cluster Theory

Dimitrios G. Liakos, Manuel Sparta, Manoj K. Kesharwani, Jan M. L. Martin, and Frank Neese J. Chem. Theory Comput. 2015, 11, 1525−1539
Contributed by +Jan Jensen

Reprinted with permission from J. Chem. Theory Comput. 2015, 11, 1525-1539.
Copyright (2015) American Chemical Society.

Two years ago Neese and co-workers published the first CCSD(T) calculation on a protein (crambin), which, according to this latest paper remains "the largest coupled cluster calculations reported to date".  The calculations were made possible through the domain based local pair natural orbital (DLPNO) implementation of CCSD(T), which includes a variety of approximations with associated thresholds that control the accuracy.

The current paper defines three default thresholds termed "LoosePNO", "NormalPNO", "and TightPNO" recommended for rapid estimates, general thermochemistry and kinetics, and non-covalent interactions and conformational equilibria, respectively.  The latter two provide relative energies with 1 kcal/mol of conventional CCSD(T) calculations.

The study includes molecules up to 30 atoms including a "real-life" organometallic catalysis and a study of 51 conformers of melatonin.  Here the focus is very much on accuracy and little or no information was given on computational cost.  So I encourage any reader with practical experience using this method to leave a comment with their experiences.

Accurate CCSD(T) benchmark data sets for small complexes has been incredibly useful in parameterizing dispersion corrections and semi-empirical methods. It looks like DLPNO-CCSD(T) has made it practical to construct similar benchmark sets for chemical reactivity of organometallic and bio-catalytic models systems which currently are too big to be treated with conventional CCSD(T).

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Using dispersion-corrected density functional theory to understand supramolecular binding thermodynamics

Antony, J.; Sure, R.; Grimme, S. Chem. Commun. 2015, 51, 1764-1774
Contributed by Steven Bachrach.
Reposted from Computational Organic Chemistry with permission

Grimme and coworkers have a featured article on computing host-guest complexes in a recentChemComm.¹ They review the techniques his group has pioneered, particularly dispersion corrections for DFT and ways to treat the thermodynamics in moving from electronic energy to free energy. they briefly review some studies done by other groups. They conclude with a new study of eight different host guest complexes, three of which are shown in Figure 1.

1	2
3

Figure 1. TPSS-D3(BJ)/def2-TZVP optimized structures of 1-3.

These eight host-guest complexes are fairly large systems, and the computational method employed means some fairly long computations. Geometries were optimized at TPSS-D3(BJ)/def2-TZVP, then single point energy determined at PW6B95-D3(BJ)/def2-QZVP. Solvent was included using COSMO-RS. The curcurbituril complex 2 includes a counterion (chloride) along with the guest adamantan-1-aminium. Overall agreement of the computed free energy of binding with the experimental values was very good, except for 3 and the related complex having a larger nanohoop around the fullerene. The error is due to problems in treating the solvent effect, which remains an area of real computational need.

An interesting result uncovered is that the binding energy due to dispersion is greater than the non-dispersion energy for all of these complexes, including the examples that are charged or where hydrogen bonding may be playing a role in the bonding. This points to the absolute necessity of including a dispersion correction when treating a host-guest complex with DFT.

As an aside, you’ll note one of the reasons I was interested in this paper: 3 is closely related to the structure that graces the cover of the second edition of my book.

References

(1) Antony, J.; Sure, R.; Grimme, S. "Using dispersion-corrected density functional theory to understand supramolecular binding thermodynamics," Chem. Commun. 2015, 51, 1764-1774, DOI:10.1039/C4CC06722C.

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Uthrene, a radically new molecule?

Melle-Franco, M. "Uthrene, a radically new molecule?," Chem. Commun. 2015, 51, 5387-5390

Contributed by Steven Bachrach.
Reposted from Computational Organic Chemistry with permission

Uthrene 1 is the smallest formally diradical fragment of graphene; it cannot be expressed in a closed shell, fully electron-paired Kekule form. Its isomer zethrene 2 on the other hand, can be expressed in closed shell form.

Melle-Franco has examined these, and related, polycyclic aromatic hydrocarbons with DFT.¹ The optimized structure of singlet and triplet 1 at CAM-B3LYP/6-31g(d,p) are shown in Figure 1. The singlet-triplet energy gap of 2 is 16.5 kcal mol^-1 with a ground state singlet. However, for 1 the triplet is predicted to be lower in energy than the singlet by 7.7 kcal mol^-1. And this gap increases to 10.9 kcal mol^-1 at CASSCF(14,14)/6-31g(d,p)//CAM-B3LYP/6-31g(d,p). Natural orbital population analysis of the singlet of 1 at CASSCF identifies two orbitals with populations around 1 e.

Interestingly, both the singlet and triplet of 1 are not planar, exhibiting a twist to avoid the clashing of the hydrogens in the bay region. (This twisting is best seen by clicking on the structures in Figure 1, and viewing the molecules interactively through Jmol.)

singlet

triplet

Figure 1. CAM-B3LYP/6-31g(d,p) optimized geometries of the singlet and triplet of 1.

References

(1) Melle-Franco, M. "Uthrene, a radically new molecule?," Chem. Commun. 2015, 51, 5387-5390, DOI:10.1039/C5CC01276G.

InChIs

1: InChI=1S/C24H14/c1-5-15-7-3-11-20-22(15)17(9-1)13-19-14-18-10-2-6-16-8-4-12-21(23(16)18)24(19)20/h1-14H
InChIKey=WAKUAPLTIMTENF-UHFFFAOYSA-N

2: InChI=1S/C24H14/c1-5-15-7-3-11-19-22-14-18-10-2-6-16-8-4-12-20(24(16)18)21(22)13-17(9-1)23(15)19/h1-14H
InChIKey=UXUXNGMSDNTZEC-UHFFFAOYSA-N

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