The large quadrupole of water molecules

Area of Science:

• Computational chemistry
• Molecular modeling
• Physical chemistry

Background:

• Common water site models underestimate molecular quadrupole moments compared to quantum mechanical calculations.
• Accurate representation of molecular multipoles is crucial for reliable simulations of water.

Purpose of the Study:

• To investigate the discrepancy in quadrupole moments between quantum mechanical calculations and common water site models.
• To identify the source of the larger quadrupole moments in quantum mechanical calculations.
• To evaluate the performance of different water models in reproducing experimental properties.

Main Methods:

• Quantum mechanical/molecular mechanical (QM/MM) calculations at MP2/aug-cc-pVQZ level.
• Comparison of multipoles from QM/MM with various site models (e.g., TIP4P, TIP5P).
• Monte Carlo simulations using a soft-sticky dipole-quadrupole-octupole multipole model.

Main Results:

• Larger quadrupole moments in QM calculations are attributed to p-orbital character and charge shifts.
• Common site models fail to capture this p-orbital electron density, leading to inaccurate moments.
• Multipole models, when using accurate multipoles, better reproduce electrostatic potentials and experimental radial distribution functions.

Conclusions:

• Standard water site models are inadequate for accurately representing the quadrupole moment of water.
• Multipole models incorporating accurate QM-derived multipoles are superior for simulating water.
• Adjusting quadrupole moments in multipole models leads to good agreement with experimental data.