Structure and potential surface of liquid methanol in low temperature: Comparison of the hydrogen bond network in methanol with water

Molecular dynamics simulations for liquid methanol have been carried out in order to examine the hydrogen bond network pattern in the low-temperature regime. Those properties of methanol concerning hydrogen bond connectivity are compared with supercooled water. Methanol can be supercooled deep into the low-temperature region without any singular behavior, which is in sharp contrast to supercooled water. One-dimensional linear hydrogen-bonded chains with occasional branches are the predominant species from room temperature to 153 K. The number of hydrogen bonds per methanol molecule in the inherent structure remains constant over a wide range of temperature. Lowering the temperature simply reduces the number of branches, keeping the total number of hydrogen bonds constant. This is caused by a decrease of the methanol molecules hydrogen-bonded with one and three other molecules. It is found that the hydrogen bond strength does not vary with temperature. The potential energy of the inherent structure decreases with a temperature decrease, suggesting that methanol falls into a category of fragile liquid. The energy decrease is due mainly to an increase in density with declining temperature, which strengthens the Lennard-Jones interaction term. This feature is distinguished from water, where hydrogen bonds become gradually stronger with decreasing temperature in the normal supercooled state.