  We performed molecular dynamics simulations of coalescence of two vacancies
in a tungsten (W) crystal to elucidate the effect of temperature and hydrogen
atoms. Simulations were performed for two types of vacancy structures,
$\mathrm{V}_9 + \mathrm{W}_1 + \mathrm{V}_9$ and $\mathrm{V}_{10} +
\mathrm{W}_4 + \mathrm{V}_{10}$ ($\mathrm{V}_{n}$ means that a vacancy
corresponds to the absence of $n$ W atoms, and $\mathrm{W}_{m}$ indicates that
there are $m$ W atoms between two vacancies) in various cases of temperature
and hydrogen atom concentration. Under the vacancy structure $\mathrm{V}_9 +
\mathrm{W}_1 + \mathrm{V}_9$, we observed vacancy coalescence for all the cases
of the temperature and the number of hydrogen atoms. Evaluating the potential
energy required for removing one of the W atoms between two vacancies, we found
that high temperature and existing hydrogen atoms in the vacancies facilitate
vacancy coalescence, and that under the structure $\mathrm{V}_{10} +
\mathrm{W}_4 + \mathrm{V}_{10}$, hydrogen atoms facilitate vacancy coalescence
most strongly when the number is around 45 to 54 in each vacancy.
