First-principles study of pressure effects on the molecular solids (CH3)4X[M(dmit)2]2 (X = N,P and M = Ni,Pd) -: art. no. 035116

We study the pressure effects on the molecular solids (CH3)(4)N[Ni(dmit)(2)](2), beta-(CH3)(4)N[Pd(dmit)(2)](2) and beta'-(CH3)(4)P[Pd(dmit)(2)](2) (dmit=2-thioxo-1,3-dithion-4,5-dithiolate) using first-principles calculations based on the density-functional theory. The atomic and electronic structures under pressure are calculated with optimizing lattice parameters. Although theoretical optimization of lattice parameters of molecular solids under pressure is difficult as they are generally soft, we show that the present approach can yield important information concerning the effects of pressure on such materials. We have found that changes of electronic structure induced by pressure are qualitatively different between Ni and Pd salts. Investigation of the two Pd salts suggests that there are sensible differences in their band structures at ambient pressure, while the pressure effects on each Pd salt are almost the same.