Nuclear quantum effects on the structure and the dynamics of [H2O]8 at low temperatures

P. E. VIDELA, P. J. ROSSKY, D. LARIA

JOURNAL OF CHEMICAL PHYSICS

AMER INST PHYSICS

Lugar: New York; Año: 2013 vol. 139 p. 134715 - 134715

0021-9606

We use ring-polymer-molecular-dynamics (RPMD) techniquesand the semi-empirical q-TIP4P/F water model to investigate therelationship between hydrogen bond connectivity and the characteristicsof nuclear position fluctuations, including explicit incorporation ofquantum effects, for the energetically low lying isomers of the prototypecluster [H$_2$O]$_8$ at T = 50 K and at 150 K. Our results reveal thattunneling and zero-point energy effects lead to sensible increments inthe magnitudes of the fluctuations of intra and intermolecular distances.The degree of proton spatial delocalization is found to map logically withthe hydrogen-bond connectivity pattern of the cluster. Dangling hydrogenbonds exhibit the largest extent of spatial delocalization and participatein shorter intramolecular O-H bonds. Combined effects from quantum andpolarization fluctuations on the resulting individual dipole momentsare also examined. From the dynamical side, we analyze the characteristicsof the infrared absorption spectrum. The incorporation of nuclear quantumfluctuations promotes red shifts and sensible broadening relative to theclassical profile, bringing the simulation results in much more satisfactoryagreement with direct experimental information in the mid and high frequencyrange of the stretching band. While RPMD predictions overestimate the peakposition of the low frequency shoulder, the overall agreement with that reportedusing an accurate, parameterized, many-body potential is reasonable, and farsuperior to that one obtains by implementing a partially adiabatic centroidmolecular dynamics approach. Quantum effects on the collective dynamics,as reported by instantaneous normal modes, are also discussed.