  Calculating excited-state properties of molecules and solids is one of the
main computational challenges of modern electronic structure theory. By
combining and advancing recent ideas from the field of quantum computing we
propose a more effective variational quantum eigensolver based on quantum
parallelism: Initial ans\"atze for various excited states are prepared into a
single pure state through a minimal number of ancilla qubits. Then a global
rotation in the targeted subspace is optimized. Our approach thus avoids the
progressive accumulation of errors prone to schemes that calculate excited
states successively. Energy gaps and transition amplitudes between eigenstates
can immediately be extracted. Moreover, the use of variable auxiliary weights
makes the algorithm more resilient to noise and greatly simplifies the
optimization procedure. We showcase our algorithm and illustrate its
effectiveness for different molecular systems. The interaction effects are
treated through generalized unitary coupled cluster ans\"atze and, accordingly,
the common unfavorable and artificial extension to the entire Fock space is
circumvented.
