Acta Physico-Chimica Sinica ›› 2020, Vol. 36 ›› Issue (11): 1908035.doi: 10.3866/PKU.WHXB201908035

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Parameterization and Validation of AMBER Force Field for Np4+, Am3+, and Cm3+

Ziyi Liu1,2, Miaoren Xia1,2, Zhifang Chai1,3, Dongqi Wang1,*()   

  1. 1 Multidisciplinary Initiative Center, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, P. R. China
    2 University of Chinese Academy of Sciences, Beijing 100049, P. R. China
    3 State Key Laboratory of Radiation Medicine and Protection, and School of Radiation Medicine and Interdisciplinary Sciences (RAD-X), Soochow University, Suzhou 215123, Jiangsu Province, P. R. China
  • Received:2019-08-29 Accepted:2019-09-24 Published:2019-09-29
  • Contact: Dongqi Wang
  • Supported by:
    the National Natural Science Foundation of China(91026000);the National Natural Science Foundation of China(21473206);the National Natural Science Foundation of China(91226105);CAS Hundred Talents Program(Y2291810S3)


The radioactivity and toxicity of actinides impede experimental investigation into their chemical properties in the condensed phase. The rapid development of computational methods and computational facilities allows for alternative experimental methods, including the use of a molecular force field, to gain insight into the coordination chemistry and dynamics of actinides. The key to this method is the force fieild parameters. In the present work, we report the development and validation of the AMBER (Assisted Model Building with Energy Refinement) force field parameters for Np4+, Am3+, and Cm3+ based on the experimentally determined ion-oxygen distance (IOD). The parameter set, together with that reported for Th4+, U4+, and Pu4+, was then applied to investigate the coordination chemistry and dynamics of these six actinide ions in the aqueous phase, in the absence and presence of counterions Cl-, NO3-, and CO32-. The simulations showed a shorter An-Ow coordination length for An4+ than for An3+, and for higher atomic numbers of ions in the same valence state. Th4+ preferentially existed in a 10-coordinated state, adopting a BCASP (bicapped square antiprism) conformation, while the other ions tended to be 9-coordinated with a CASP (capped square antiprism) conformation. The only exception was Cm3+, which adopted a TCTP (tricapped trigonal prism) conformation. The results also showed that the water molecules around An4+ were more ordered than those around An3+, as indicated by the smaller angles between the An-Ow vector and the dipole direction of the water ligand. This highly ordered structure of coordinated water affected their translation and rotation, i.e., the diffusion coefficient and rotational relaxation time of the water molecules around An4+ were smaller than those in the case of An3+ due to the stronger electrostatic interaction between An4+ and ligating water. The hydration free energies of the targeted actinide ions were also calculated by the FEP (free energy perturbation) method. An4+ underwent a greater degree of stabilization than did An3+ upon hydration; among the ions in the same oxidation states, those with a higher atomic number were better stabilized. In summary, the results of the simulations were consistent with the literature data in terms of the hydration structure, coordination of counterions, and hydration free energy of the actinide ions. The ability of the parameter set to describe the dynamics of water in the vicinity of actinides remains to be verified due to the lack of reference data. We tentatively propose that it may be used to investigate the coordination chemistry of actinides both in conformational analysis and binding strength, while special care should be taken when studying the kinetics of the solvated system. This work is expected to enrich our understanding of the solution behavior of An3+/An4+ at the force field level.

Key words: An3+, An4+, AMBER force field


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