Basic aspects of the charge density wave instability of transition metal trichalcogenides NbSe₃ and monoclinic-TaS₃

NbSe$_3$ and monoclinic-TaS$_3$ ($m$-TaS$_3$) are quasi-1D metals containing three different types of chains undergoing two charge density wave (CDW) Peierls transitions at T$_{P_1}$ T$_{P_2}$. The nature these is discussed on the basis first-principles DFT calculation their electron-hole Lindhard response function. As a result stronger inter-chain interactions Fermi surface (FS) function considerably more complex than those for $m$-TaS$_3$; however common scenario can be put forward to rationalize results. intra-chain inter-band nesting processes dominate strongest both type I III compounds. Two well-defined maxima found with (0$a$*, 0$c$*) (1/2$a$*, 1/2$c$*) transverse components T$_{P_2}$, respectively, whereas second maximum not observed $m$-TaS$_3$ T$_{P2}$. Analysis coupling mechanisms leads conclusion that FS effects only relevant set $a$* in NbSe$_3$. For along $c$* $m$-TaS$_3$, one must take into account Coulomb mechanism. Phonon spectrum calculations show formation giant 2$k_F$ Kohn anomaly $m$-TaS$_3$. All results support weak transition metal trichalcogenides.