Harmonic

Quantum mechanics

Rotor (electric)

Potential energy surface

Anharmonicity

Density functional theory

Coupled cluster

Basis (linear algebra)

Atomic physics

Ab initio

Molecule

Computer science

Rigid rotor

Scaling

Basis set

Physics

Organic chemistry

Cluster (spacecraft)

Mathematics

Molecular physics

Maxima and minima

Molecular dynamics

Mathematical analysis

Computational chemistry

Binding energy

Geometry

Chemistry

Programming language

Harmonic oscillator

The Journal of Physical Chemistry A

For (H2O)n where n = 1–10, we used a scheme combining molecular dynamics sampling with high level ab initio calculations to locate the global and many low lying local minima for each cluster. For each isomer, we extrapolated the RI-MP2 energies to their complete basis set limit, included a CCSD(T) correction using a smaller basis set and added finite temperature corrections within the rigid-rotor-harmonic-oscillator (RRHO) model using scaled and unscaled harmonic vibrational frequencies. The vibrational scaling factors were determined specifically for water clusters by comparing harmonic frequencies with VPT2 fundamental frequencies. We find the CCSD(T) correction to the RI-MP2 binding energy to be small (<1%) but still important in determining accurate conformational energies. Anharmonic corrections are found to be non-negligble; they do not alter the energetic ordering of isomers, but they do lower the free energies of formation of the water clusters by as much as 4 kcal/mol at 298.15 K.

Benchmark Structures and Binding Energies of Small Water Clusters with Anharmonicity Corrections