Hello, A new project on the LCRC cluster has been requested. Please forward the information on to the LCRC Allocation sub-committee. Applicant's name: Pierre Darancet Applicant's institution: ANL Applicant's division: NST Project Name: IVwire Project title: Length dependence of the current-voltage characteristics of molecular wires Associated funding: Other Systems: CNM Carbon (200,000h) Science: Using non-equilibrium Green's functions methods (NEGF), we will compute the electronic structure and current response of polymers submitted to an external bias, and understand how such response is impacted by the length of the molecule. Using our recently-developed approach [1], our goals are (1) to achieve a quantitative description of the current-voltage characteristics of polyphenylene vinylene (PPV) measured by our collaborators at Columbia University and (2) to develop a minimal physical model predicting the lineshape and the conditions of appearance of non-linearities in such characteristics. This model will be used to design polymers with tailored electronic and transport properties. Project description: We will study the finite-bias electronic transport properties of single-molecule junctions --devices consisting in an individual molecule connected to macroscopic metallic electrodes. Experimental breakthroughs in the past 10 years allow to statiscally and reproducibly measure the current-voltage characteristics (IVs) emerging from such devices and to quantify the different energy scales impacting the way electrons flow at the sub-nanometer lengthscale. Among other things, these developments have led to the development of a quantitative description of such IVs characteristics based on first-principles calculations [1]. The ongoing theoretical and experimental challenge is to use this understanding in order to design molecular junctions with new functionalities, i.e. with non-linear (non-Ohmic) IVs. As transport at low-bias in these systems typically varies exponentially with length, we intend in this project to understand if (and how) the size of the molecule can be used to g enerate strongly non-linear IV characteristics. We have recently developed a bias-dependent parameter-free self-energy correction to density functional theory (DFT) that allows for a quantitative description of the current-voltage characteristics of molecular junctions [1]. This method is implemented on top of NEGF-DFT implemented in the freely-available Siesta package [2,3]. This software was already used on the CNM cluster to compute the low-bias current response of PPV molecules with a number of monomers ranging from 1 to 5: showing an excellent agreement between our results and experiments conducted by our collaborators at Columbia University. The next step is to compute the full IV characteristics and requires significantly more computational power. Siesta is efficiently parallelized for orbitals-based operation as well as number of points used to analytically continue the Green's functions to the real axis: All finite-bias calculations are done on a Keldysh contour (~100 points near the Fermi energy at 1V) and involve the explicit description of ~500-700 atoms/2000-3000 electrons. Siesta parallelization involves a 1-D block cyclic distribution for orbital-based operations, with an optimal number of orbitals per cpu in the 10-32 range (24 was optimal on Carbon). As we anticipate the scaling will deteriorate when the number of cores exceed the number of Keldysh contour points, we will use 96 cpus (16 nodes) per run. Based on the cpu time on Carbon, we anticipate that each run will require 200h of wall time. A minimum 4 voltages/molecule (5 molecules total) will be needed to quantify the non-linearity. Siesta 3.1.0 is already installed and supported on the LCRC clusters. Nevertheless, in order to implement our self-energy correction, we will need to recompile the code --which requires the use of standard libraries, available on blues/fusion: MPI/BLACS/Scalapack. [1] Darancet et al. Nano Letters 12, 6250-6254 (2012) [2] Soler et al., J. Phys.: Condens. Matter 14, 2745 (2002) [3] Brandbyge et al., Phys. Rev. B 65, 165401 (2002). Industry partnership: Project URL: Requested allocation: 400000 Q1: 0 Q2: 0 Q3: 0 Q4: 400000 Justification: Storage requirements: The requester has used undetermined amount hours of their initial startup project. In addition to approving an initial amount, please specify a Category and Subcategory for this project. For a list of the current selection of approved categories, please see: https://wiki.lcrc.anl.gov/wiki/Processes/Categories Once the Allocation committee has approved the project, please go to the Project Management page to create it: https://accounts.lcrc.anl.gov/projects.php Thank You, The LCRC Accounts System