how to call the atom style variables for calculation

Submitted by dixonarulraj on Tue, 12/10/2013 - 16:39

i want to calculate the volume change according to the change in the diameter of the particles but
i cant able to use the atom style variables in the calculation.

i cant able to use the variable radi for the formulation of the volume and i am getting an error saying
ERROR: Per-atom compute in equal-style variable formula (variable.cpp:895)

variable vtotal equal 1^3
variable vp1 equal ${radi}^3)*3.1415/4
print ----- ${vp1} -----
variable n1 equal (${vtotal}-${vp1})/${vtotal}
print "## quit"

please help !
Thanks in advance