Atomic partial charges and one Lennard-Jones parameter crucial to model cellulose allomorphs

The influence of the non-bonded parameters, i.e., Lennard-Jones and the partial atomic charges, on the predicted unit cell dimensions of different allomorphs of cellulose were studied in the framework of the GROMOS force field. Systematic variation of partial atomic charges revealed the particular importance of charge distribution at the proximity of glycosidic linkage to the monoclinic angles. Furthermore, the unit cell parameters were better predicted when the repulsive term of the united atom CH1 (carbon atoms bearing one hydrogen) was optimized. The a-axis of cellulose I_β was over estimated by more than 7 and 8.3 % in GROMOS-53A6 and GROMOS-56A_carbo respectively, but gave prediction within 0.2 % from experimental value, i.e. within experimental accuracy, when the CH1 repulsion term was optimized and CHARMM charge set was imported. At the same time, the average deviation from experimental values of the lattice parameters of four allomorphs was improved from 2.36 to 1.18 % for GROMOS-53a6 and from 2.53 to 1.75 % for GROMOS-56A_carbo.