The excitation property of P700 has been studied using the time-dependent density functional method. It is found that the excitation of P700 is intrinsically asymmetric at its two Chla halves, and this asymmetry may be closely related to the relative use of two electron transfer branches. Based on the calculated results, we also proposed a charge separation mechanism for P700, in which the time evolution of the excited state of P700 is modulated by a quantum beat of period about 75 fs, making the electron transfer of two branches correlated, or complementary effect possible. (c) 2006 Elsevier B.V. All rights reserved.