Periodic density functional theory and local-MP2 study of the librational modes of Ice XI

被引:39
作者
Erba, Alessandro [1 ]
Casassa, Silvia
Dovesi, Roberto
Maschio, Lorenzo
Pisani, Cesare
机构
[1] Univ Torino, Dipartimento Chim IFM, I-10125 Turin, Italy
关键词
ab initio calculations; density functional theory; HF calculations; librational states; vibrational modes; INCOHERENT NEUTRON-SCATTERING; LOCALIZED WANNIER FUNCTIONS; AB-INITIO; VIBRATIONAL-SPECTRA; MOLECULAR-DYNAMICS; ENERGY; CRYSTAL; RANGE; IH; FREQUENCIES;
D O I
10.1063/1.3076921
中图分类号
O64 [物理化学(理论化学)、化学物理学];
学科分类号
070304 ; 081704 ;
摘要
Two periodic codes, CRYSTAL and CRYSCOR, are here used to simulate and characterize the librational modes of the nu(R) band of Ice XI: this band has been found experimentally to be the region of the vibrational spectrum of ordinary ice most affected by the transition from the proton-disordered (Ice Ih) to the proton-ordered (Ice XI) phase. With CRYSTAL, the problem is solved using Hartree-Fock (HF), pure Kohn-Sham (PW91) or hybrid (B3LYP) one-electron Hamiltonians: the harmonic approximation is employed to obtain the vibrational spectrum after optimizing the geometry. The B3LYP results are those in best agreement with the experiment. For a given crystalline geometry, CRYSCOR computes the energy per cell in an ab initio HF+MP2 approximation using a local-correlation approach; this technique is employed for recalculating the frequencies of the different modes identified by the B3LYP approach, by fully accounting for long range dispersive interactions. The effect of anharmonicity is evaluated separately for each mode both in the B3LYP and HF+MP2 case. The two approaches accurately reproduce the four-peak structure of the librational band. The harmonic B3LYP nu(R) bandwidth of 70 meV is lowered to 60 meV by anharmonic corrections, and becomes 57 meV in the HF+MP2 anharmonic calculation, in excellent agreement with the experimental IINS data (56-59 meV). The assignment of the librational modes is discussed.
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页数:9
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