A computer model of a one-dimensional carbon wire gave different electron-transport patterns depending on how its orbitals were treated, according to an arXiv preprint. The study reported its most reliable qualitative transport results with CASSCF-optimized orbitals rather than CASCI calculations using delocalized DFT orbitals.
The work used the NEGF-MCPDFT framework to calculate electron transport in carbon wires of various lengths joined to aluminum electrodes.
A junction built for comparison
The modeled junction had six aluminum atoms in each electrode, spaced 2.53 angstroms apart, with a carbon wire in the central region. The model used aluminum rather than gold.
The researchers compared fixed-orbital CASCI calculations with CASSCF calculations that re-optimized the orbitals. They chose orbitals near the gap between the highest occupied and lowest unoccupied molecular orbitals, including orbitals with density at the electrode-wire interface. The active space contained six electrons in six orbitals for even-carbon wires and eight electrons in eight orbitals for odd-carbon wires.
Even and odd wires behaved differently
The calculated structures followed a parity pattern: even-carbon wires were more acetylenic, while odd-carbon wires were more cumulenic. In the CASCI results, most even-length wires had a gap in transmission near the approximate Fermi level, a semiconducting-like pattern. The exception was a very narrow peak near zero energy for the two-carbon chain.
Odd-length wires produced a different CASCI picture. They showed narrow transmission peaks beginning near the Fermi level, across a reported range of 0.0 to 0.6 electronvolts, and the overlap with the Fermi level increased as the wire grew longer. The pattern was consistent with conducting behavior in the modeled systems.
Changing the orbital treatment changed the transmission pattern. CASSCF produced no peaks overlapping the Fermi level for even-length wires, but more Fermi-level overlap than CASCI for odd-length wires, with that overlap increasing with length.
The voltage and length tests exposed gaps
Across the explored bias range of 0.1 to 0.5 volts, both CASCI and CASSCF produced alternating conductance patterns between even- and odd-length wires. The pattern appeared only at higher voltages in the CASCI calculations, while CASSCF showed it at all the explored voltages. The authors noted that the result could be sensitive to how the Fermi level was aligned.
The calculations also did not reproduce the exponential fall in conductance with length reported for the experimental comparison. Odd-length conductance generally did not decay exponentially, while the even-length CASCI and CASSCF results were low but variable rather than following that pattern.
A one-shot DFT-like calculation, using a trivial active space of two electrons in one orbital and no self-consistent density optimization for the electrode-wire coupling, gave another result. Its alternating pattern did not appear until 0.5 volts and ran opposite to the experimental direction, with even-length chains showing higher conductance; at low voltage, conductance fell with system length.
What the authors say should improve the models
The authors recommend an active space that captures multiconfigurational character, meaning that several electron arrangements need to be considered, in both the electrodes and the central wire. They present CASSCF orbital optimization as a useful way to do that, even though its optimized orbitals became localized on the terminal electrodes.
The finding applies to the aluminum-carbon junctions modeled in this work. The calculations did not reproduce the cited exponential conductance decay for even-length chains, and low-bias conductance was sensitive to Fermi-level alignment. Even- and odd-carbon systems also used different active-space sizes, a design choice that complicates direct comparisons.
The paper is an arXiv version 1 preprint dated 25 August 2026.
Paper data and sources
Original title: Exploring electron transport in carbon wire systems using a wave function-based multiconfigurational non-equilibrium Green's function approach
Authors: Pranesh Raghavendran, Gabriela E. Campbell, Erik P. Hoy, Andrew M. Sand
Journal/Repository: arXiv
Status: Preprint, not yet peer-reviewed
First online: 2026-08-25
DOI: Not available
Original paper · Full text