$opertemp is the reference temperature in the program. Usually it is set as Ambient temperature
Example
$opertemp
300.0
This means the reference temperature is 300K.
The $opertemp setting in GUI interface is here
Press Contact Setting and fill in the field for the Temperature(K)!
![2D opertemp fig1.jpg](/mediawiki/images/thumb/7/71/2D_opertemp_fig1.jpg/1200px-2D_opertemp_fig1.jpg)