Hi In the spc.itp file, Is the HEAVY_H = Deuterium ? Then, H2O become D2O ? Thank you Lin #ifdef _FF_GROMOS96 #ifdef HEAVY_H 1 OW 1 SOL OW 1 -0.82 9.95140 2 H 1 SOL HW1 1 0.41 4.03200 3 H 1 SOL HW2 1 0.41 4.03200 #else 1 OW 1 SOL OW 1 -0.82 15.99940 2 H 1 SOL HW1 1 0.41 1.00800 3 H 1 SOL HW2 1 0.41 1.00800 #endif