We present benchmark binding energies of naturally occurring gas molecules CH 4 , CO 2 , and H 2 S in the small cage, namely, the pentagonal dodecahedron (5 12 ) (H 2 O) 20 , which is one of the constituent cages of the 3 major lattices (structures I, II, and H) of clathrate hydrates. These weak interactions require higher levels of electron correlation and converge slowly with an increasing basis set to the complete basis set (CBS) limit, necessitating the use of large basis sets up to the aug-cc-pV5Z and subsequent correction for basis set superposition error (BSSE). For the host hollow (H 2 O) 20 cages, we have identified a most stable isomer with binding energy of −200.8 ± 2.1 kcal/mol at the CCSD(T)/CBS limit (−199.2 ± 0.5 kcal/mol at the MP2/CBS limit). Additionally, we report converged second order Møller−Plesset (MP2) CBS binding energies for the encapsulation of guests in the (H 2 O) 20 cage of −4.3 ± 0.1 for CH 4 @(H 2 O) 20 , −6.6 ± 0.1 for CO 2 @(H 2 O) 20 , and −8.5 ± 0.1 kcal/mol for H 2 S@(H 2 O) 20 , respectively. For CH 4 @(H 2 O) 20 , exhibiting the weakest encapsulation affinity among the three, we report CCSD(T)/aug-cc-pVTZ binding energies and, based on them, a CCSD(T)/CBS estimate of −4.75 ± 0.1 kcal/mol. To the best of our knowledge, the CCSD(T)/aug-cc-pVTZ calculation for CH 4 @(H 2 O) 20 is the largest one reported to date (168 valence electrons, 1978 basis functions, and the correlation of 84 doubly occupied and 1873 virtual orbitals) and required a scalable implementation of the (T) module on 6144 nodes (350 208 cores) of the "Cori" supercomputer at the National Energy Research Supercomputing Center (NERSC) for a total execution time of 195 min (for the (T) part). These efficient scalable implementations of highly correlated methods offer the capability to obtain long-lasting benchmarks of intermolecular interactions in complex systems. They also provide a path toward parametrizing classical potentials needed to study the dynamical and transport properties in these complex systems as well as assess the accuracy of lower scaling electronic structure methods such as density functional theory (DFT) and MP2 including its spin-biased variants.

We assess the performance of 7 pairwise additive (TIP3P, TIP4P, TIP4P-ice, TIP5P, OPC, SPC, SPC/E) and 8 families of many-body potentials (q-AQUA, HIPPO, AMOEBA, EFP, TTM, WHBB, MB-pol, MB-UCB) in...

We present a classical electrostatic induction model to evaluate the 3-body Ion-Water-Water (I-W-W) and (W-W-W) interactions in aqueous ionic systems. The monatomic ions were described by a point charge and a dipole-dipole polarizability, while for the polyatomic ions distributed multipoles up to hexadecapole and distributed polarizabilities up to quadrupole-quadrupole were used. The accuracy of the classical model is benchmarked against an accurate dataset of 936 (I-W-W) and 2,184 (W-W-W) 3-body terms for 13 different monatomic and polyatomic cation and anion systems. The classical model shows excellent agreement with the reference MP2 and CCSD(T) 3-body energies. The Root-Mean-Square-Errors (RMSEs) for monatomic cations, monatomic anions, and polyatomic ions were 0.29 kcal/mol, 0.25 kcal/mol, and 0.12 kcal/mol, respectively. The corresponding RMSE for 1,744 CCSD(T)/aVTZ 3-body (W-W-W) energies, used to train MB-pol, was 0.12 kcal/mol. The accuracy of the classical model demonstrates that the 3-body term for aqueous ionic systems can be accurately modeled classically, without the need to fit to tens of thousands of high-level ab initio calculations. This approach provides a fast but accurate and efficient path towards modeling the 3-body effect in aqueous ionic systems that is fully transferable across systems with different ions.

We report a Many Body Energy (MBE) analysis of aqueous ionic clusters containing anions and cations at the two opposite ends of the Hofmeister series, viz. the kosmotropes Ca2+, SO42-...