Deformation gradient F and Stretch tensor U in USER material routines

Dear all,

I have a question about the Deformation gradient F and Stretch tensor U in USER material routines. Do F and U already exist in Radioss routines? Because when I write a linear isotropic elastic material subroutine, I don't need to use deformation gradient F. But when I develop hyperelastic materials using USER material routines, I need to use deformation gradient F. Do I only need to declare F like this in engine subroutine, and then I can use them to calculate stress tensor directly because they already exist and are calculated automatically in Radioss?

       DOUBLE PRECISION      .FPSXX(NEL),FPSYY(NEL),FPSZZ(NEL),      .FPSXY(NEL),FPSYX(NEL),FPSXZ(NEL),      .FPSZX(NEL),FPSYZ(NEL),FPSZY(NEL)

Here is a slide from a tutorial of Radioss User Subroutines:<?xml version="1.0" encoding="UTF-8"?>

Besides, for 'Extended User Material Laws', in Chapter 3 of 'RADIOSS User's Code Interface', the data structure USERBUF doesn't contain FPSXX, FPSYY, FPSZZ etc. Which confuses me is how to use Deformation gradient F when write Radioss usermat subroutines? More precisely, if F already exists in Radioss routines, I only need to declare them and use them directly?

<?xml version="1.0" encoding="UTF-8"?>

Could someone answer my question? Any help would be appreciated.

Best