I used the Forcite module to calculate the young's modulus of a supercell of the lattice structure of polyvinyl alcohol matrix embedded with nano diamond (mass ratio=1%). I used the default dettings: force field=universal, geometric optimisation algorithm= smart and all other settings were maintained default. After carrying out the geometric optimization, energy and dynamic calculations, i used the 'mechanical properties' tool to calculate its youngs modulus. It came out to be:

x:27.1330GPa

y:360.5738GPa

z:-59.6581GPa

wheras only for PVA (without the nano diamond)and using the same settings, the youngs modulus was:

x:13.8752GPa

y:318.67GPa

z:7.4131GPa

Moreover, when i used the same method to evaluate the mechanical properties for the nano composite of PVA and CNT, both the y and z components of the young's modulus were negative!

What do the negative values in the nano composite imply? What type of force field should I use, or what changes should I make to the default parameters to obtain accurate results for a nano composite? Please help me out on this matter. Moreover, I would like to know the criterion and the factors one needs to know while deciding the force field or the type of geometric parameterisation algorithm one has to use (especially for nano composites),

Thank you.