Montecarlo variation from file in AMS simulator

I am trying to simulate a montecarlo adding mismatch to some ideal components (each wrapped within a cell). To do that I add in the model libraries a file like this:

statistics {

mismatch {
vary MyPar dist=gauss std=MyPar_sigma
}

}

it all works fine as long as I am using spectre, but when I turn to ams simulator there is no variation anymore. Is it a mater of different syntax?

How can I model mismatch in a similar way in ams simulator?

Furthermore in trying this out, when setting the simulator in the same ADE assembler view back to spectre (after having tried with ams simulator), upon running again a simulation I get the following error:

Error found by spectre during circuit read-in.
ERROR (SFE-868): "input.scs" 535: Cannot open the input file 'partial/path' because either the file name or the directory specified is invalid. Ensure that the specified file exists and the path to the file is valid. Alternatively, use the -I <path> command-line option to specify the path to the file and rerun the simulation.

Where partial/path is the beginning of a path pointing to some model files. it is not clear to me why, the last line of input.scs is include "partial/path". My only guess is that somehow switching to ams simulator something like this was added somewhere in some settings automatically and remained there after going back to spectre? How can I fix this? basically all assembler views give this error after returning back to spectre if they have been set to ams simulator at some point...

Thank you

Best regards