---
author:
- 'Hitoshi <span style="font-variant:small-caps;">Seo</span>$^{1,2}$[^1], Shoji <span style="font-variant:small-caps;">Ishibashi</span>$^{3}$, Yuichi <span style="font-variant:small-caps;">Otsuka</span>$^{4}$, Hidetoshi <span style="font-variant:small-caps;">Fukuyama</span>$^{5,6}$, and Kiyoyuki <span style="font-variant:small-caps;">Terakura</span>$^{3,7}$'
title: ' Electronic States of Single-Component Molecular Conductors \[$M$(tmdt)$_2$\] '
---

Introduction {#sec1}
============

Molecular crystals composed of one molecular species showing electrical conduction, i.e., single-component molecular conductors (SCMC) [@Tanaka_2001Science; @Kobayashi_2004CR], have been revealed to bear novel electronic states. In particular, their multiorbital nature has been recognized, which is different from the situation in conventional charge transfer salts (CTS), where, in most cases, only one molecular orbital (MO) contributes to their electronic properties [@Seo_2004CR; @Review_2006JPSJ]. In fact, the involvement of different MO is a consequence of the molecular design [@Kobayashi_2001JMC] for realizing SCMC: to make the energy difference between the frontier MO small enough, so that their energy bands can overlap when inter-molecular transfer integrals become sufficiently large. Metal complex molecules of the form $M$($L$)$_2$ ($M$ = metal, $L$ = ligand) are suitable for this purpose. Their frontier MO are approximately bonding and antibonding combinations of the $p\pi$ wave functions from the two ligands. Large ligands lead to an effectively small transfer integral between them and result in a small energy difference. Such a situation indicates a two-MO system, which is 