Hello and sorry for my English

I try to make a simulation for the rompe and weizel law , who has this equation:

The d=0.000766 and the a=0.00015

I try to simulate that with a G source and write the equation like that

{0.000766/(SQRT(2*0.00015*(STD(PWR(I(R3),2)))))}

(STD because i saw that the integral in pspice is STD in a manual that i found)

but its an error, specifically :

ERROR -- Expression - use previously defined parameters only

What is wrong with that;

The I(R3) in the current from the all ready existing resistance.