RMintexp is a univariate stationary covariance model
depending on a univariate variogram model $\phi$.
The corresponding covariance function only depends on the difference
$h$ between two points and is given by
$$C(h)=(1 - exp(-\phi(h)))/\phi(h)$$
Schlather, M. (2012)
Construction of covariance functions and unconditional simulation of
random fields.Lecture Notes in Statistics, Proceedings,207, 25--54.