Characteristics of instantaneous resonant modes in simple dense fluids with short-ranged repulsive interactions

We manifest the characteristics of the low-frequency, quasilocalized instan taneous normal modes, named as the instantaneous resonant modes (IRMs), in simple dense fluids with short-ranged repulsive interactions. The analyses include the potential energy profiles of the IRMs, and the local geometric structures and the number of the interacting neighbors of the particles at which the centers of the quasilocalization are located. We conclude that an IRM is created due to fluctuations in the local density, and has a barely- isolated center, which slightly interacts with one or two nearest neighbors , and the potential energy profile of an IRM is basically single-well with strong anharmonicity. The differences in character between the IRMs and the high-frequency localized instantaneous normal modes are also examined. Bas ed on the barely isolated center picture, a necessary criterion for the occ urrence of the IRMs is proposed. While only the imaginary-frequency IRMs ar e found in dense fluids with purely repulsive interactions satisfying the c riterion, a tiny attractive well in the pair potential allows the occurrenc e of the real-frequency IRMs. The physical systems to detect the presence o f the IRMs are discussed. (C) 2000 American Institute of Physics. [S0021-96 06(00)50225-6].