  We modify the NonLinear Guiding Center (NLGC) theory (Matthaeus et al. 2003)
for perpendicular diffusion by replacing the spectral amplitude of the
two-component model magnetic turbulence with the 2D component one (following
Shalchi 2006), and replacing the constant $a^2$, indicating the degree
particles following magnetic field line, with a variable $a^{\prime 2}$ as a
function of the magnetic turbulence. We combine the modified model with the
NonLinear PArallel (NLPA) diffusion theory (Qin 2007) to solve perpendicular
and parallel diffusion coefficients simultaneously. It is shown that the new
model agrees better with simulations. Furthermore, we fit the numerical results
of the new model with polynomials, so that parallel and perpendicular diffusion
coefficients can be calculated directly without iteration of integrations, and
many numerical calculations can be reduced.
