10 Importance of Molecular Crystals . . . . . . . . . . . . . . . . . . . . . . . . . . 199
10.1 Motional Correlation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199
10.1.1 Significance and Difficulty . . . . . . . . . . . . . . . . . . . . . 199
10.1.2 Entropic Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . 200
10.1.3 Dynamics in Quasi-one-dimensional Systems . . . . . . . . 202
10.2 Molecular Crystals as Tunable Model System . . . . . . . . . . . . . . 204
10.2.1 Basis of Tunability . . . . . . . . . . . . . . . . . . . . . . . . . . . 204
10.2.2 Unified Description of Structural Phase
Transitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205
10.2.3 Impurity Effects on Structural Phase Transitions . . . . . . 208
10.3 Molecular Crystals as Stage for Novelties . . . . . . . . . . . . . . . . 210
10.3.1 Coupling Between Molecular Dynamics
and Electronic System . . . . . . . . . . . . . . . . . . . . . . . . 210
10.3.2 Phenomena Involving Molecular Flexibility . . . . . . . . . 211
10.3.3 Dipolar Ising System . . . . . . . . . . . . . . . . . . . . . . . . . 211
10.3.4 Exotic Superstructure . . . . . . . . . . . . . . . . . . . . . . . . . 212
10.3.5 Molecular Aggregation in a Gyroid Phase . . . . . . . . . . 214
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 218
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