多体相互作用对高压固氦状态方程的影响

采用量子化学自洽场方法及原子团簇理论对固氦晶格原子势能进行多体展开，定量确定了最近邻原子间距R在0.26—0.175nm之间，短程的两体到五体相互作用对晶体结合能及压缩特性的贡献，并对不同间距下多体展开式的截断位置进行了讨论. 结果表明：当最近邻原子间距R在0.26—0.175 nm之间取值，在具有hcp相结构的晶格中，氦原子势能的多体展开式是一收敛的交错型级数，该级数中两体项、四体项为正值，而三体项、五体项为负值. 当R在0.26—0.23 nm之间取值时, 仅保留级数中两体和三体项就能很好地描述晶格原子势能；当R在0.23—0.20 nm之间取值，多体展式需要展开到四体相互作用项；当R在0.20—0.175 nm之间取值必须再加入五体项贡献. 考虑到五体相互作用后，理论计算已能圆满地解释目前固氦等温压缩数据，其最大压力达 60GPa. 关键词： 固氦 状态方程 多体相互作用

Many-body contributions to the equation of state for highly compressed solid helium

To investigate the significance of various many_body effects on the equation of state (EOS) of solid helium,the interaction energy of hcp crystal structure is expanded as a sum of two_ to five_body potentials in the present work. Each short_range many_body term between helium atom clusters is computed by means of Hartree_Fock (HF) self_consistent_field (SCF) method. It is indicated that at high densities the many_body expansion formula is an alternative series, in which two_, and four_body terms are positive, whereas three_ and five_body terms are negative. According to such an expansion of crystal interaction energy, the equation of state of solid helium at 0 K is obtained. The specific characteristics of each of the up to five_body contributions in highly compressed helium are discussed. Between 0.26 to 0.23nm, the interaction energy may be well described by two_ and three_body interactions. Over a distance range 0.23—0.20nm, truncated many_body expansion after the four_body term recovers the main part of atomic potential energy. Below 0.20nm the five_body contributions have to be included. The sum of two_ to five_body term accurately reproduces the atomic potential energy from 0.20 to 0.175nm. The inclusion up to five_body potential satisfactorily describes the available experimental data from 7.5 to 2.5 cm³/mol, corresponding to 1—60 GPa.