  The interaction of ultrashort laser pulses with thin tungsten and tantalum
films is investigated through the full-potential band-structure calculations.
Our calculations show that at relatively low absorbed energies (the electron
temperature $T_e$$\lesssim$7 kK), the lattice of tantalum undergoes noticeable
hardening. The hardening leads to the change of the tantalum complete melting
threshold under these conditions. Calculations suggest that for the
isochorically heated Ta film, if such hardening really occurs, the complete
melting threshold will be at least 25% higher. It is also shown that the
body-centered cubic structures of W and Ta crystals become dynamically unstable
when the electronic subsystem is heated to sufficiently high temperatures
($T_e$$>$22 kK). This lead to their complete melting on the sub-picosecond time
scale.
