ML in R (caret-package) missing hyperparameters

I have a pretty specific question regarding the caret package however I still hope to finde help here.

I recently worked with the caret package and trained a multilayer perceptron with method = 'mlp'.
I looked up the github page of Max Kuhn (developer of caret), and it says that you only need to tune one hyperparameter: the size (number of neurons in the hidden layer). Which is really convinient.

However it further states that caret for the training builds on the RSNNS Package (by Bergmeier). The mlp model implemented in this RSNNS package has additional tunable parameters over just the size hyperparameter (i.e. learnFunc,hiddenActFunc,Std_Backpropagation, maxit).

So I asked myself what values caret uses for those parameters? Default values or are those optmizied?

It appears that the defaults are used, except for lin, which is inferred from the type of the target variable: [source code]
• @SysRIP, which hyperparameters to tune is certainly another question (probably already asked); but on topic here, I think that caret having deemed all the other defaults as good enough to not include as parameters indicates that size is a good enough start. RSNNS indicates where you might go next, with "The defaults that are set for initialization and update functions usually don't have to be changed": that leaves activations, learning function, number of iterations. RSNNS seems to be missing some modern options, e.g. RELU activation and adam optimizer? – Ben Reiniger Dec 13 '19 at 14:53