One thing you can never do is over commit available RAM in the machine. If the application server fails to start and SystemOut.log contains a message like this:
JVMJ9VM015W Initialization error for library j9gc23(2): Failed to instantiate heap. 1G requested
Could not create the Java virtual machine.
Then it is highly likely that you tried to start an app server with not enough free physical RAM. Check the "free -m" command on Linux. Otherwise refer to your OS specific manuals to see how to determine how much free RAM you actually have.