One standard answer is this: An atheist can love God under a a description different from standard theistic ones (cf. this article). For instance, God is the truth, and an atheist might love the truth.
But there is a second story that could perhaps be told. St. Augustine said that to love someone, one must know the beloved to exist. This is at least a little too strong, I think. One might believe but not know that the beloved exists. But even that might be too strong. Suppose that George believes there is no life after death and knows his wife, Patricia, has died. Could we not say that, nonetheless, his love for his wife could survive her death? Love is a matter of will, not intellect. Certainly, as long as George thinks Patricia is alive he can love her. But a mere change of belief is not a change of will. It seems, thus, that unless there is a shift in his will, he can continue to love Patricia even after she is dead and even though he does not believe she is alive. Moreover, it does not seem right to say that George is just loving the past Patricia in some eternalist (or growing block) sense. Besides, one could tell a story where George comes to believe that Patricia never existed--maybe he comes to believe that he had always hallucinated her. But if love is a matter of will, not intellect, then he can continue to love Patricia even after he acquires this belief.
If so, then an atheist could love God. But, likely, not every atheist does. (Just as, likely, not every theist does.) Some atheists would be relieved to learn God exists (J.J.C. Smart sounds that way in his volume with John Haldane), but some atheists might be rather dismayed.
Even if it is possible to love someone whom one believes not to exist, it is not clear that one can love someone who doesn't exist. Love is of a particular individual, and there is typically[note 1] no way to individuate totally non-existent beings.