Direct calculations of crystalline thermal expansion and molecular reorientation from nonbonded interatomic potential anharmonicity and thermal amplitudes

Non-bonded interatomic potential functions were altered to allow for the effects of vibration and anharmonicity by relating the mean displacement to the mean-square vibrational amplitude through the anharmonicity constant, K. Vibrational amplitudes were calculated in the harmonic approximation. The repulsive coefficient, B, of the potential was increased sufficiently to cause the minimum of the potential to shift by the amount of the displacement. The apparent lattice energy was then minimized in the usual manner. The method was checked against experiment by calculation of the lattice constants of noble gases at several temperatures, and of anthracene at two temperatures. The method differs from usual calculations of the Grüneisen type in that calculation of molecular-orientation change with temperature is possible. The anthracene molecular orientation was calculated at 95 and 290°K.