The java command has several options that influence the runtime behaviour, see the documentation of your Java installation. These options have to be specified before the `-jar` option. E.g., for the Java runtime environment of Sun, the option `-Xmx` specifies the maximum amount of heap memory size that should be allocated. The default value of this option may be much smaller than your installed main memory size. In this case GroIMP cannot benefit from your memory until you specify the `-Xmx` option as in

java -Xmx400m -jar path/to/core.jar

which allows a maximum heap memory allocation of 400 MB. If you encounter `OutOfMemoryError`s when running GroIMP, you have to specify this option with a larger value. Note that the opposite may happen, i.e. a too generous memory allocation might prevent GroIMP from starting at all. In that case you have to lower the alocated memory again until it works.