The development of accurate multi-component molecular theory for quantum simulation of hydrogen-bonding and positronic systems

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Recently, we have been developing some new first-principles approaches which treat multi component systems including both electrons and nuclei(positron)quantum-mechanically. To include the new many-body contribution such as electron-nuclei and nuclei-nuclei correlations, we have developed(1)multi-component density functional theory (MC DFT) and (2) multi-component quantum Monte Carlo (MC QMC)methods.1.Development of MC DFT: We have defined Kohn-Sham operators for multi-component DFT (MC DFT) ,and derived analytical equations for total energy and gradient with respect to classical nuclear coordinates. Using MC DFT method, we have analyzed the H/D isotope effect of porphine and porphycene molecules with electron correlation, and confirmed that our results are in reasonable agreement with the corresponding experimental values. In the HD-species which replaced an inner-hydrogen to a deuterium, such high-symmetric geometries are broken down and the induced electronic polarization drasticall … More y changes the dipole moment values. These results clearly indicate that the difference of quantum nature between proton and deuteron influences the electronic structure and molecular geometry. Tb improve accuracy of our MC (HF+DFT) method, we have also developed a new electron-nucleus correlation functional based on the Colle-Salvetti correlation functional.2.Development of MC QCM: In order to analyze the positron-molecular compounds with quantitative accuracy, we have developed the multi-component quantum Monte Carlo (MC QMC) method, and applied to the positronic compounds of[H: e+]1 and [HCN: e^+]systems. Although the variational Monte Carlo calculation gives poor results in the case of [H: e^+] system, the diffusion Monte Carlo result is in good agreement with that by Hylleraas-type wave function. Our result of -0.788949 (82) hartree is much lower than that using the full-configuration interaction method with one-centered Gaussian type basis functions. In the case of [HCN: e^+]system, to our knowledge, the most accurate energy is reported as -92.901915 hartree in 2006 by Strasburg. On the other hand, we have obtained -93.2591 (5) and -93.39830 (8) hartree by VMC and DMC method, and achieved the WORLD RECORD variational energy! Less