  We investigate the effect of coherence to the excitation transfer efficiency
(ETE) in photosynthetic bacteria Chlorobium Tepidum. We have modeled the
monomer of Fenna-Matthews-Olson (FMO) complex as consisting of eight
bacteriochlorophyll-a sites, while explicitly consider reaction center core
complex (RCC) as an additional site. With the use of realistic bath spectrum
and several dominant vibronic modes in the non-Markovian master equation, in an
effective 9-site model, we have compared the ETE for an initial pure state and
an initial mixed state. We observe that the initial pure state relaxes
efficiently to increase the trapping at the RCC. We further illustrate that the
coherence play a competitive role to block the back transfer of excitation from
RCC pigment to FMO complex and hence to maximize the ETE.
