Our previous analysis of the effect of a constant (Onsager reaction) electric field on a formic acid molecule showed that it produced an elongating force on the O-H bond resulting from the parallelism of field and dipole derivative, leading to a red-shifting of the frequency and an increase in infrared band intensity with the field, and a contracting force on the C-H bond, resulting from the antiparallelism in the above quantities, leading to a blue-shifting of frequency and an initial decrease, followed by an increase, in band intensity with the field. In this paper we extend this analysis to the characterization of the O-H...O and C-H...O hydrogen bonds in the nonconstant fields present in the formic acid dimer. We use a QM/MM treatment,that incorporates exchange-repulsive forces and we also include forces generated by intramolecular cross-term interactions. The excellent agreement with the ab initio bond length changes as a function mainly of the balance of the electrical and repulsive forces demonstrates that the same physical forces can account for the different structural and spectroscopic behavior of these types of hydrogen bonds, thus not requiring a fundamental distinction to be made between them.