How to find or measure dVi/dt of input signal in HSPICE and Current Source Model

Hi all..

I am trying to pre characterize a cmos inverter through current source models in HSPICE. For that i need to implement some equations.

One of the equation is

i0+ I(Vi,Vo)-Cm(Vi,Vo)dVi/dt=0

I found the i0 and I(Vi,Vo), now i need to find Cm....but before that i have to find dVi/dt of input signal....i tried it thru .measure deriv command of HSPICE. But couldnt succeed.
I am posting code here...

*code for slewrate characterization

vdd 3 0 1v
vdd1 4 0 1v
vch1 1 0 pwl(0 0 2n 0 10n 1v 100n 1v 110n 0)
vch2 6 0 0.1v
v1 2 6 0v
m1 2 1 3 3 pmos l=2u w=90u
m2 2 1 0 0 nmos l=1u w=45u
m3 5 6 4 4 pmos l=2u w=360u
m4 5 6 0 0 nmos l=1u w=180u

.lib "45nm_model" cmos_models

.tran 1n 100n sweep vch2 0 1v 0.1v

.MEAS tran s DERIV vch1 from=1ns to=100n


.plot
.probe

.option post
.end

Please help me out in this matter.....and if anybody has any idea about implementation of current source model..then please guide me....

Thanks for your time...

Regards

Sapna