A quantum mechanics/molecular mechanics molecular dynamics simulation was performed for liquid water to investigate structural and dynamical properties of this peculiar liquid. The most important region containing a central reference molecule and all nearest surrounding molecules (first coordination shell) was treated by Hartree-Fock (HF), post-Hartree-Fock [second-order Moller-Plesset perturbation theory (MP2)], and hybrid density functional B3LYP [Becke's three parameter functional (B3) with the correlation functional of Lee, Yang, and Parr (LYP)] methods. In addition, another HF-level simulation (2HF) included the full second coordination shell. Site to site interactions between oxygen-oxygen, oxygen-hydrogen, and hydrogen-hydrogen atoms of all ab initio methods were compared to experimental data. The absence of a second peak and the appearance of a shoulder instead in the gO-O graph obtained from the 2HF simulation is notable, as this feature has been observed so far only for pressurized or heated water. Dynamical data show that the 2HF procedure compensates some of the deficiency of the HF one-shell simulation, reducing the difference between correlated (MP2) and HF results. B3LYP apparently leads to too rigid structures and thus to an artificial slow down of the dynamics.
Refractive index (RI) measurements, depolarized collision-induced light (CILS) scattering and ab initio quantum chemical calculations are used to determine the dipole (α), dipole–quadrupole (A), and dipole–octopole polarizability (E) of adamantane, C10H16. For this molecule of symmetry group Td the three polarizabilities can be represented by a single scalar quantity. From experiment we obtain for the static dipole polarizability at T≈400–500 K 107.5±1.1 e2a02Eh−1, and for the higher polarizabilities |A|=102.0±7.8 e2a03Eh−1, and |E|=720±80 e2a04Eh−1. We have performed conventional ab initio and density functional theory calculations with specifically designed basis sets. A very large [4s3p3d1f/3s2p1d] basis set consisting of 574 basis functions is thought to provide near-Hartree–Fock values for α,A and E: α=101.72 e2a02Eh−1, A=−6.5 e2a03Eh−1, and E=−71.0 e2a04Eh−1. Our final theoretical estimates for these properties are α=107.5±1.0 e2a02Eh−1, A=−8.0±1.5 e2a03Eh−1, and E=−76.5±5.5 e2a04Eh−1. Very strong electron correlation effects are found for both the first (β) and second (γ) hyperpolarizability. Our estimate for β≡βxyz and the mean γ̄ are 41.4±5.6 e3a03Eh−2 and (25±2)×103 e4a04Eh−3, respectively. For the octopole and hexadecapole moments we propose Ω=−4.4±0.2 ea03 and Φ=−100.8±5.5 ea04. The basis sets constructed in this work should provide reliable computational tools for the study of intermolecular interactions of adamantane.
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