CICE6.4.0 updates CICE6.3.1 from February 2022, and includes Icepack1.3.2. This release discretizes the dynamics equations (momentum and stress) for a C-grid. The grid infrastructure was generalized and enhanced to allow coupling among ice, ocean and atmosphere on A- (cell centered), B- (CICE default), C- (vector components located on cell faces) and CD-grids (vectors located on cell faces; under development). The new C-grid capability is now being tested in coupled modeling systems so that needed adjustments can be made for future releases. The modifications in this release are bit-for-bit in standalone CICE configurations but not when coupled. This release also deprecates a few older features of the code, including the 0-layer thermodynamics and the CESM melt ponds. In this version, the code is deprecated using preprocessor flags to enable the changes to be more easily reversed, if necessary. Please contact the CICE Consortium with any concerns regarding deprecated code. We will consider keeping it. Additional, non-bit-for-bit enhancements identified with the C-grid work will be included in the next major release, as will finalization of the deprecated code features (i.e., removal of the preprocessor flags). Major Changes C-grid implementation including infrastructure support #715 #719 #726 #728 #729 #744 deprecate 0-layer thermodynamics #732 deprecate CESM ponds #733 Bug fixes None Enhancements refine scripts #723 #731 #745 update machine files #725 update mct/cesm1 driver #726 add unit test for optional arguments #730 update Icepack #743, #748 update the CICE qc t-test plotting implementation #742 Documentation recommended value of ndte #722 Update version and copyright date #748