We investigated the fundamentals of the effect of C addition on Na flux GaN growth by first-principles calculation. We simulated C-added Na-Ga melts using molecular dynamics (MD) simulations to examine the local melt structure around a N atom. We also calculated C-N bond energy using constrained MD simulations. Results show that a N atom bonded to a C atom and there were no Ga atoms around the N atom because C-N bond energy was larger than Ga-N bond energy. This is the reason for the suppression of heterogeneous nucleation by C addition. It was also found that the C-N bond energy was affected by surrounding Ga atoms and that the C-N atomic distance increased with the Ga coordination number around the N atom. (C) 2013 The Japan Society of Applied Physics