Geometry optimization of Nanoparticle TiO2

[Resear20]

Dear Quantumatk users
I built a TiO2 nanoparticle like sphere from anatase phase using wurlff constructor. After that I optimized it using force field calculator. As a result, the optimized nanoparticle structure was completely distorted or deformed from the original structure.
               
My question, if there is any option that can reduce this distortion.
Because, I found in many published papers that the optimized structure of TiO2 NP is not distorted like I obtained.
Thank you for your help

[mlee]

Can you check the reference Forcefield paper? If it is for a bulk, it will be different in NP.
Another option is that you can add the constraint such as rigid or fix in the core part. It maybe change the distortion.
But the Force field parameters will be the main cause in this case.   

[Resear20]

Dear ATK users
Thank you for your reply
I checked the reference of forcefield and it is for nanoparticle: S. Kim, A. C. van Duin, and J. D. Kubicki, Molecular dynamics simulations of the interactions between TiO2 nanoparticles and water with Na+ and Cl-, methanol, and formic acid using a reactive force field, Journal of Materials Research, 28, p. 513, 2013.
I also used LCAO calculation and the result of optimization for TiO2 NP is the same as forcefield calculation.

Best regards