Vinayak Vipradas
Ansys Employee

The initial values of the coefficients can come from examining the function that defines the model to which you are attempting to compare/fit your data. For most hyperelastic models, 1 or -1 is a good starting point; however, coefficient values can vary greatly depending on the model chosen. The Gent model, for example, provides good fit with initial coefficient values as high as 1000.

For further information, please refer to the following link: 

https://ansyshelp.ansys.com/Views/Secured/corp/v222/en/ans_mat/Hlp_G_STRmcfhyp.html#hmcfinitialize