An investigation of the electrostatic energy in the formic acid crystal, using multipole and point‐charge lattice sums

The electrostatic part of the lattice energy of the formic acid crystal is calculated using multipole moments up to the sixth moment and point charges obtained from ab initio wavefunctions for formic acid monomers. The calculations are carried out as a function of the molecular orientation in the crystallographic unit cell. It is found that the electrostatic lattice energy of formic acid is both large in magnitude (−17.8 kcal/mole) and varies strongly with the molecular orientation. Several orientations, including the one observed experimentally, minimize the electrostatic energy and the location of these minima is dictated by nearest‐neighbor interactions. The nonobserved minima can be excluded for geometrical reasons. The electrostatic component of the lattice energy is found to act as a driving force for molecular distortion upon crystallization. Qualitative but not quantitative agreement is obtained between the multipole lattice energy and the point‐charge energy based on Mulliken populations.