The effect of molecular vibrations on calculated second virial coefficients

Abstract

The limitation of the rigid molecule approximation in the calculation of the second virial coefficient is examined using two different potential models for steam. Both the MCY and the ST2 potentials have been augmented by allowing bond flex and stretch according to an ab initio internal potential. Second virial coefficients are calculated within the classical approximation by the Monte Carlo method. The calculated coefficients with flexible molecules are lower than in the rigid case by up to 10% for the MCY potential and 8% for the ST2 potential.

The pressure second virial coefficient of vibrating symmetric triatomic molecules

“…The Metropolis scheme was then used to either accept or reject the move. The results obtained using this scheme agreed with those using the scheme of Refson et al [2]. However, we found in general that the result converged somewhat faster, usually in between 5 and 7 million steps.…”

“…We found it rather puzzling that Refson (0.5-2) x 10 6 configurations, because the model we have used appears simpler than that used in [2]. However, we were demanding a higher accuracy than the 0.5% achieved in [2], since the contributions to B due to vibrations themselves are of a similar order of magnitude in our models, whereas they were larger in [2] (see section 4). It is also possible that for a harmonic potential, convergence is slower.…”

“…As a check on our calculations, we also developed our own MC scheme. The six intermolecular integrations were carried out using crude MC as in [2]. For the intramolecular integrations, we performed a Metropolis sampling of the internal modes, by starting from the equilibrium or some other suitable positions, and allowing the atoms to move such that 161/aoo1 ~< 0.0025, [6z/aoo[ <<.…”

“…Our results showed that the contributions were too small to be of significance in the deduction of intermolecular potentials from second-virial-coefficient data. In view of the rather large contributions reported for water by Refson et al [2] from internal vibrations, we report here a study of these effects for triatomic symmetric molecules. Our objective was to determine if these large contributions are uniquely large for water only, or for triatomics in general.…”

Preliminary observations on a new water-water potential

“…The quality of the MCY two-body potential was tested by computing various properties of liquid water in Monte Carlo [ 41 and molecular dynamics [ 5,6] simulations; other tests, such as the calculation of the second [ 7,8] and third [ 91 virial coefficients have also been made for the gas state.…”