  In a quantum computer the hardware and software are intrinsically connected
because the quantum Hamiltonian (or more precisely its time development) is the
code that runs the computer. We demonstrate this subtle and crucial
relationship by considering the example of electron-spin-based solid state
quantum computer in semiconductor quantum dots. We show that multielectron
quantum dots with one valence electron in the outermost shell do not behave
simply as an effective single spin system unless special conditions are
satisfied. Our work compellingly demonstrates that a delicate synergy between
theory and experiment (between software and hardware) is essential for
constructing a quantum computer.
