[13msmemusman]

Hye i am new user of openFoam i need to calculate double derivative with respect to time of a scalar.
i have a scalar CoMx
i tried to find its double derivative by

scalar DDTCoMx = d2dt2(CoMx);

but i got error
error: ‘d2dt2’ was not declared in this scope
scalar DDTCoMx = d2dt2(CoMx);

please help me... is there any need to add some header file explaining d2dt2???? if yes then where this header file lies and how to add it in make/options???