vocab : marxism, capitalism, socialism, communism

To "say capitalism" or "say socialism", is to "do Marxism", because the latter simply refers to a linguistic framework that includes concepts such as capitalism and socialism. 

To "do capitalism", is to accumulate capital, which is what all living things do, in the state of nature (evolved, thus far).

To "do socialism", is to distribute capital, which is also something that many but not all living things do, in the state of nature (evolved thus far).

So, capitalism and socialism are not opposites, but different modes of capital distribution. To do Marxism, is to study such distributions of capital. 

You might then, ask if there is then no distinction between Marxism and economics in general. It would be coherent then to say, that Marxism is simply a variety of economics characterised by concepts of capitalism and socialism, whereas there are varieties of economics which do not employ those concepts. 

Communism is thus trivial : it's just the set of texts encapsulating the practical politics of Marx and Engels, both of whom can be understood as Marxist theorists.