The effect of external fields on the structural and electronic properties of Ag(l 10) is investigated using local-density-functional theory. We find that Ag(l 10) undergoes a (1x2) missing-row reconstruction as an excess charge of « 0.05 electron per surface atom is added onto the surface. Our result supports an electron-donation mechanism for the alkali-metals-induced reconstruction on the (110) surface of the 3d and Ad fee metals. The surface interlayer spacings are found to be insensitive to the applied field in the absence of field-induced surface reconstruction.

Ding, Chan, and Ho Reply:It is indeed unfortunate that, in our Letter, we missed the important paper by Johansson, Landemark, Karlsson, and Uhrberg [l] (JLKU), which reported a careful study of the surface band structure and the symmetry properties of the (V3xV3)Ag/Si(lll) surface using polarization-dependent angle-resolved photoemission. They clearly established that the (VJx VJ) surface has a mirror plane containing the surface normal and the [112] direction and the existence of a highly dispersive surface state (S\) with its minimum close to the Fermi level at the f point in the surface Brillouin zone. These observations offer strong support for the HCT-1 model [2] that we have found for the (VJx V3)Ag/Si(l 11) surface via firstprinciples total-energy calculations. The HCT-1 model, as described in our paper [2], does have the mirror plane and the £1 surface state found in the experiment. These points have already been emphasized in the preceding Comment [3].

The vibrational frequencies of three C84 fullerene isomers are calculated with a tight-binding potential model. The differences between the vibrational properties of these isomers are discussed.Following the breakthrough in producing macroscopic quantities of C6o, ' a number of higher fullerenes have also been recently synthesized.The study of structural and dynamical properties of these higher fullerenes has attracted considerable theoretical as well as experimental interests. Among the higher fullerenes, C84 is one of the more abundant species. Although experimental e8'orts have produced detailed information on the ' C nuclear magnetic-resonance spectrum, infrared (IR) spectrum, and ultraviolet photoemission spectrum of C84, ' ' ' many chemical and physical properties of this molecule remain unclear. Three isomers of C84 with Td, D6I"and helical D2 symmetries first proposed by Fowler have been widely studied ' and the helical D2 isomer was preferred by simple Hiickel theory. However, more recent calculations' ' showed that the helical D2 isomer is, in fact, energetically very unfavorable. In our previous work' based on tight-binding calculations for all 24 isolated-pentagon isomers of C84, ' we found that two isomers with D2 and D2d symmetries are energetically most stable. Moreover, these two isomers are so close in energy and structure that they may form an inseparable mixture in the synthesis of C84. Similar results were also obtained from modified neglect of difFerential overlap calculations' and first-principles total-energy calculations. i4'~6In this paper, we present a study of the vibrational properties of the low-energy Dz and D2d isomers of C84 using a tight-binding potential. We also studied the lessstable helical D2 isomer for comparison. Although several theoretical studies on the vibrational properties of C84 have been reported, " the calculations have been performed for the less-stable T", D6~, and helical D2 geometries and not for the low-energy D2 and D2d structures. The tight-binding (TB) potential model used in the present calculation is taken from the previous work of Xu et al. ' In this model, the total energy contains two terms, of various structural and dynamical properties of carbon fullerenes, including the predictions of low-energy structures of C84 and the study of vibrational properties of 18 C6o.The structures of the two low-energy isomers and the helical D2 isomer of C84 used in the present calculation are shown in Fig. 1. The coordinates of the isomers have been optimized by the tight-binding potential. As one can see from Fig. 1, the two low-energy D2 and D2d isomers are quite spherical while the helical D2 isomer has an elongated shape. It is also interesting to note that the two low-energy isomers can be transformed from one to (a) D2 (b) D2d (c) DZ(Helical) occupied E(Ir, I)= g (P"~II&B(Ir;I)~P")+E"&(Ir,j) . The first term in (I) is the electronic energy calculated by a parametrized TB Hamiltonian HrB(Ir; I), and the second term is a short-ranged repulsive energy. This tight-b...