title: Interstellar radical species binding on ices: a hybrid QM/MM approach

authors: W. M. C. Sameera,* Bethmini Senevirathne, Muhsen Al-Ibadi, Stefan Andersson, Gunnar Nyman

abstract: In hybrid quantum mechanics/molecular mechanics (QM/MM) methods, the electronically most important region (e.g. binding site, reaction center) can be described by a QM method, while a MM description can be used for the remaining part that acts in a perturbative fashion.[1,2] Therefore, QM/MM methods provide accurate results at low computational cost. However, hybrid QM/MM methods are not popular in modeling chemical processes on ices. This is due to limited availability of the force fields in most practical QM/MM implementations. We have developed the "Shell Interface for Combining Tinker With ONIOM" (SICTWO) program to overcome this limitation.[3-5] SICTWO supports several polarizable and non-polarizable force fields that are suitable for modeling ices with the ONIOM(QM:MM) method. On the practical side, we have combined the AMOEBA polarizable force field with the ONIOM approach to rationalize binding energies and binding preference of the interstellar radical species (OH, HCO, CH3) and atoms (O and H) on crystalline water ice (Ih). Our results suggested that the dangling-hydrogen (d-H) and dangling-oxygen (d-O) of the binding sites play an important role on the binding energies and binding preference.[5] The binding energy is stronger in the presence of both d-H and d-O at the binding site, and this dangling nature is typical for the "Striped phase".[6,7] On the other hand, radical binding become relatively weaker in the presence of three d-H or three d-O at the binding site, which are common for the proton disordered form of the crystalline water ice (Ih) surfaces. The outcome of this work provides important insights of the roles of d-H and d-O in adsorption of radical species on interstellar ices. 


References
[1] W. M. C. Sameera, F. Maseras, WIREs Comput Mol Sci, Wiley VCH, 2012, 2, 375. 
[2] L. W. Chung, W. M. C. Sameera, R. Ramozzi, A. J. Page, M. Hatanaka, G. P. Petrova, T. V. Harris, X. Li, Z. Ke, F. Liu, H-B. Li, L. Ding and K. Morokuma, Chem. Rev. 2015.
[3] W. M. C. Sameera, F. Maseras, Phys. Chem. Chem. Phys., 2011, 13, 10520. 
[4] I. Rivilla, W. M. C. Sameera, E. Alvarez, M. M. Daz-Requejo, F. Maseras, P. J. Prez, Dalton Trans, 2013, 42, 4132-4138. 
[5] W. M. C. Sameera, B. Senevirathne, S. Andersson, G. Nyman (Submitted).
[8] N. H. Fletcher, Philos. Mag. 1992, 66, 109-115. 
[7] V. Buch, H. Groenzin, I. Li, M. J. Shultz, E. Tosatti, PNAS, 2008, 105, 5969-5974.