nor gate power in hspice

[diod]

hi
i need tocalculate power with hspice for nor2 gate and estimate power my self.hspice power value is 8.2902E-06 and my estimate with this formula=(vdd^2*c)/2=(3.5^2*50)/2=3.6502e-4.in xor2 and nand2 i calculate power they were near to hspice power simulation.i dont khow my formula for power is wrong?why in nor it is more diffrence?
it is my nor gate code in hspice:
cmos-nor
vdd vdd gnd 3.5
mpa out a x vdd pch W=10u L=1u
mpb x b vdd vdd pch W=10u L=1u
mna out a gnd gnd nch W=10u L=1u
mnb out b gnd gnd nch W=10u L=1u
cload out gnd 50fF
va a gnd pulse 0 3.5 1n 100p 100p 1ns 4ns
vb b gnd pulse 0 3.5 3n 100p 100p 4ns 6ns
.tran 100p 10ns
.print tran v(out)
.MEASURE TRAN avgpwr AVG POWER FROM=1ns TO=3ns
.PARAM vvddpar=3.5
.measure TRAN iavg AVG i(mnb) FROM=1ns TO=3ns
.measure TRAN power PARAM='iavg*vvddpar'
.lib 'all_mos.l' tt
.end