Modeling and Simulation of Charge Dynamics in Molecular Field-Coupled Nanocomputing Circuits

Molecular Field-Coupled Nanocomputing (MolFCN) stands out as a promising solution to overcome the limitations posed by CMOS scaling. MolFCN technology implements the Quantum Cell Automa (QCA) paradigm, in which the information is encoded within the so-called quantum dots of a unit cell. The information is then propagated between unit cells by electrostatic interaction. Among beyond-CMOS technologies, MolFCN stands out for its miniaturization, low-power operation, and potential to achieve hundreds of gigahertz switching frequencies at room temperature. In MolFCN, information is encoded and propagated through electrostatic intermolecular interactions guided by a multi-phase clocking mechanism, which ensures correct signal propagation and pipelining. The Self-Consistent Electrostatic Potential Algorithm (SCERPA) has been introduced to simulate a clocked MolFCN circuit. However, SCERPA does not include charge dynamics and its effect on information propagation and elaboration. Prior research enabled the modeling of the intramolecular charge movement using Real-Time Time-Dependent Density Functional Theory (RT-TDDFT), permitting the molecules to be approximated as first-order low-pass filters in the dynamic domain. This thesis explores modeling the dynamic intramolecular interaction during information propagation by modeling the circuit as a chain of low-pass filters defined by molecule-specific cutoff frequencies. The modeling and simulation are applied to butane and bis-ferrocene molecules and begin with calculating molecular chain voltages considering the electrostatic interactions. Time is discretized with the Finite Element Method (FEM), and the forward Euler method is used to solve the dynamic voltage equation, reflecting non-stationary inputs from neighboring molecules' charge redistributions. Charge distribution is determined using the Voltage-Dependent Atomic Charge Transfer (VACT) method, which links potential differences to charge profiles based on the molecule type. Multiple VACT profiles exist for bis-ferrocene, varying with clock field polarization. The simulative setup models transient behavior and measures propagation times through molecular wires and majority voters, verifying results against expected molecular switching frequencies. Longer chains or increased molecular spacing cause signal loss, consistent with theoretical predictions. This work extends SCERPA to enable dynamic MolFCN circuit simulations, opening new ways of simulating signal propagation dynamics in MolFCN circuits for the first time.