Thanks to Marc for spotting the issue!

index par by names does not work.

set.seed(13); x <- rnorm(1000, 5, 2)

negll <- function(par, x){ -sum(dnorm(x=x, mean=par["mean"], sd=par["sd"], log=TRUE)) }

optim(par=c(mean=1, sd=1), fn=negll, x=x, method = "L-BFGS-B", lower=c(-Inf, .0001))

The same code cannot be used in optim and optimParallel

library(optimParallel)

cl <- makeCluster(2) # set the number of processor cores

setDefaultCluster(cl=cl) # set 'cl' as default cluster

optimParallel(par=c(mean=1, sd=1), fn=negll, x=x, method = "L-BFGS-B", lower=c(-Inf, .0001))

ERROR