We develop an efficient method to generate an accurate semiglobal potential energy surface of a molecule in condensed phases with low computational cost. We apply the method to the calculation of the site energies and their fluctuations of bacteriochlorophyll (BChl) a pigments in the Fenna-Matthews-Olson (FMO) complex using the density functional properly describing the ground and excited states of BChl a in solutions in our previous work (J. Phys. Chem. B 2014, 118, 10906-10918). The errors of the potential energies calculated from the present and QM/MM methods are small: similar to 1 kcal/mol for both the ground and excited states. The calculated site energies are in good agreement with the experimentally fitted results. The calculated spectral density also agrees with the experimentally available data. The spectral densities of BChl 2 and BChl 5 are much larger than those of the other five sites. The present method is expected to provide new insights into the efficient excitation energy transfer in light-harvesting antennas.