Strak Paweł, Krukowski Stanisław
Faculty of Physics, Warsaw University of Technology, 00-672 Warsaw, Koszykowa 75, Poland.
J Chem Phys. 2007 May 21;126(19):194501. doi: 10.1063/1.2733651.
Quantum mechanical (QM) high precision calculations were used to determine N(2)-N(2) intermolecular interaction potential. Using QM numerical data the anisotropic potential energy surface was obtained for all orientations of the pair of the nitrogen molecules in the rotation invariant form. The new N(2)-N(2) potential is in reasonably good agreement with the scaled potential obtained by van der Avoird et al. using the results of Hartree-Fock calculations [J. Chem. Phys. 84, 1629 (1986)]. The molecular dynamics (MD) of the N(2) molecules has been used to determine nitrogen equation of state. The classical motion of N(2) molecules was integrated in rigid rotor approximation, i.e., it accounted only translational and rotational degrees of freedom. Fincham [Mol. Simul. 11, 79 (1993)] algorithm was shown to be superior in terms of precision and energy stability to other algorithms, including Singer [Mol. Phys. 33, 1757 (1977)], fifth order predictor-corrector, or Runge-Kutta, and was therefore used in the MD modeling of the nitrogen pressure [S. Krukowski and P. Strak, J. Chem. Phys. 124, 134501 (2006)]. Nitrogen equation of state at pressures up to 30 GPa (300 kbars) and temperatures from the room temperature to 2000 K was obtained using MD simulation results. Results of MD simulations are in very good agreement (the error below 1%) with the experimental data on nitrogen equation of state at pressures below 1 GPa (10 kbars) for temperatures below 1800 K [R. T. Jacobsen et al., J. Phys. Chem. Ref. Data 15, 735 (1986)]. For higher temperatures, the deviation is slightly larger, about 2.5% which still is a very good agreement. The slightly larger difference may be attributed to the vibrational motion not accounted explicitly by rigid rotor approximation, which may be especially important at high temperatures. These results allow to obtain reliable equation of state of nitrogen for pressures up to 30 GPa (300 kbars), i.e., close to molecular nitrogen stability limit, determined by Nellis et al. [Phys. Rev. Lett. 53, 1661 (1984)].
采用量子力学（QM）高精度计算来确定N(2)-N(2)分子间相互作用势。利用QM数值数据，以旋转不变形式获得了氮分子对所有取向的各向异性势能面。新的N(2)-N(2)势与范德阿沃德等人利用哈特里-福克计算结果得到的标度势[《化学物理杂志》84, 1629 (1986)]相当吻合。利用N(2)分子的分子动力学（MD）来确定氮的状态方程。N(2)分子的经典运动在刚性转子近似下进行积分，即仅考虑平动和转动自由度。芬奇姆[《分子模拟》11, 79 (1993)]算法在精度和能量稳定性方面优于其他算法，包括辛格[《分子物理学》33, 1757 (1977)]、五阶预测-校正算法或龙格-库塔算法，因此被用于氮压力的MD建模[S. 克鲁科夫斯基和P. 斯特拉克，《化学物理杂志》124, 134501 (2006)]。利用MD模拟结果得到了压力高达30 GPa（300 kbar）以及温度从室温到2000 K时氮的状态方程。对于温度低于1800 K、压力低于1 GPa（10 kbar）的情况，MD模拟结果与氮状态方程的实验数据非常吻合（误差低于1%）[R. T. 雅各布森等人，《物理化学参考数据杂志》15, 735 (1986)]。对于更高的温度，偏差稍大一些，约为2.5%，但仍然吻合得很好。稍大的差异可能归因于刚性转子近似未明确考虑的振动运动，这在高温下可能尤为重要。这些结果使得能够获得压力高达30 GPa（300 kbar）时可靠的氮状态方程，该压力接近内利斯等人[《物理评论快报》53, 1661 (1984)]所确定的分子氮稳定性极限。
