Why will OpenDJ and Tomcat not run simultaneously without crashing?

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http://serverfault.com – I have an OpenDJ directory server running on a VPS system. As soon as I start Tomcat, however, the OpenDS server dies within a few minutes, with the error Java HotSpot(TM) 64-Bit Server VM warning: Exception java.lang.OutOfMemoryError occurred dispatching signal SIGTERM to handler- the VM may need to be forcibly terminated in the logs (repeated about 40 times each crash). When they are both running, free -m shows at least 1970mb of free memory (the total is 3906mb) I am not hugely familiar with Java virtual machines, so can anyone point me in the direction of what is happening?? (HowTos)