  Quantum-dot cellular automata (QCA) may provide low-power, general-purpose
computing in the post-CMOS era. A molecular implementation of QCA features
nanometer-scale devices and may support THz switching speeds at
room-temperature. Here, we explore the ability of molecular QCA circuits to
tolerate unwanted applied electric fields, which may come from a variety of
sources. One likely source of strong unwanted electric fields may be electrodes
recently proposed for the write-in of classical bits to molecular QCA input
circuits. Previous models have shown that the input circuits are sensitive to
the applied field, and a coupled QCA wire can successfully transfer the input
bit to downstream circuits despite strong applied fields. However, the ability
of other QCA circuits to tolerate an applied field has not yet been
demonstrated. Here we study the robustness of various QCA circuits by
calculating their ground state responses in the presence an applied field. To
do this, a circuit is built from several QCA molecules, each described as a
two-state system. A circuit Hamiltonian is formed and diagonalized. All
pairwise interactions between cells are considered, along with all
correlations. An examination of the ground state shows that these QCA circuits
may indeed tolerate strong unwanted electric fields. We also show that circuit
immunity to the dominant unwanted field component may be obtained by choosing
the orientation of constituent molecules. This suggests that relatively large
electrodes used for bit write-in to molecular QCA need not disrupt the
operation of nearby QCA circuits. The circuits may tolerate significant
electric fields from other sources, as well.
