Communism is not the opposite of capitalism.
Communism is a mental or spiritual disorder due to which which greedy, fearful, envious, groveling, bootlicky slave class NPCs empower tyrannical totalitarian fascists to assume absolute power and feel noble about it. Communism is the most certain enabler of fascism. All parties in the United States (GOP or Dems) or in the world (left-wing or centrist or right-wing), all Constitutions and all States today - the very root and foundation of modern democracy is communistic and fascistic, differing only by degree. In terms of values and culture, all of the left, all independents and moderates and almost all of the right are oriented this way. Broadly, one can simply also call this Statism.
Another enabler of fascism is cultural Marxism (neo-Marxism). This used to be the hallmark of the progressive woke left. Recently, big corporations and corporate religion inc. have adopted it.
One might therefore identify these forces together as Marxist fascism.
The opposite of that is anarchism and voluntaryism.
Under an anarchic and voluntaryist social order, a libertarian legal order can logically arise and will likely probably arise almost everywhere within, due to the extreme cultural impetus toward it and the cultural resistance against violations of it. Libertarianism (the private property norm) is THE only ethically correct moral legal theory.
Free markets can only exist in a libertarian legal order.
Capitalism, if it refers to the fact that people sometimes save some of their produce for future investment or consumption, is a rather unremarkable, redundant or incidental word, referring to probable natural actions in a free market. The free market today exists only in small pockets, known as the gray or black markets. Capitalism has nothing to do with the modern crony corporatist economy. (The latter is sometimes called "capitalism" by the elites, and "anarcho-communists" historically arose in opposition to that and some currently still do. When they have and when they do, they can rightly be referred to as anarchists and the "communism" they espouse has nothing to do with the Fascist communism described above, and reduces down to an insignificant incidental, just like the word "capitalism").