In this paper, a novel method for fixed-node quantum Monte Carlo is given.
By comparing this method with the traditional fixed-node one, this novel me
thod can be applied to calculate molecular energy more exactly. An expansio
n of the eigenvalue of the energy for a system has been derived. It is prov
ed that the value of the energy calculated using the traditional fixed-node
method is only the zeroth order approximation of the eigenvalue of the ene
rgy. But when using this novel method, in the case of only increasing less
computing amounts ( < 1%), the rust order approximation, the second order a
pproximation, and so on can be obtained conveniently withr the detailed equ
ations and steps in the practical calculation to calculate the values of th
e zeroth, first and second approximation of the energies of 1 (1)A(1) state
of CH2, (1)A(g) (C-4h, acet) state of C-8 and the ground-states of H-2, Li
H, Li-2, and H2O The results indicate that for these states it needs only t
he second order approximation to obtain over 97% of electronic correlation
energy, which demonstrates that this novel method is very excellent in both
the computing accuracy and the amount of calculation required.