That got my attention - info here at the Wiki site:
http://en.wikipedia.org/wiki/Negative_calorie_ diet with a list of fruit and veg you can go for - the cooking will add calories, but try sprinkling lots of fresh herbs for added flavour at no calorie cost, so it is really a salad time for you....
Cabbage soup diet plan is not a good option as it is designed for morbidly overweight patients to use prior to operations - yes it will drop weight quickly but I trust you are aware that your hospital will provide a meal plan for you?
Take professional advice as the calorie amount you state is not a healty intake...I note you say that you have been advised this by a hospital, but I hope this takes into account your general health.