• BASE - Bielefeld Academic Search Engine (Open Access)
  BASE is one of the world's most voluminous search engines especially for academic open access web resources. BASE is operated by Bielefeld University Library
  http://www.base-search.net/

• ChemSpider
  ChemSpider is a free chemical structure database providing fast text and structure search access to over 30 million structures from hundreds of data sources
  http://www.chemspider.com/

• CiteSeerx (Open Access)
  Citeseerx is a scientific literature digital library and search engine that focuses primarily on the literature in computer and information science. Citeseerx aims to improve the dissemination of scientific literature and to provide improvements in functionality, usability, availability, cost, comprehensiveness, efficiency, and timeliness in the access of scientific and scholarly knowledge
  https://citeseerx.ist.psu.edu/

• CORE
  CORE (Connecting Repositories ) aims to facilitate free access to scholarly publications distributed across many systems. As of today, CORE gives you access to millions of scholarly articles aggregated from many Open Access repositories
  http://core.kmi.open.ac.uk/search 

• Google Scholar
• Google Scholar is a freely accessible web search engine that indexes the full text of scholarly literature across an array of publishing formats and disciplines
  http://www.google.com/schhp?hl=en

• Lens.org
  It allows users to Search, Analyzes and Manage Patent and Scholarly Data
  https://www.lens.org/

• OALib: Open Access Library
  OALib has a Search Engine, a Journal, an Index, and a Repository with OALib PrePrints and external preprints and postprints, all kept in one of the 322 OALib Disciplinary Repositories.
  http://www.oalib.com/

• PubChem
  is an open chemistry database at the National Institutes of Health (NIH). “Open” means that you can put your scientific data in PubChem and that others may use it. Since the launch in 2004, PubChem has become a key chemical information resource for scientists, students, and the general public.
  https://pubchem.ncbi.nlm.nih.gov/

• PubMed (Partially Open Access)
  PubMed comprises more than 34 million citations for biomedical literature from MEDLINE, life science journals, and online books. Citations may include links to full-text content from PubMed Central and publisher web sites
  http://www.ncbi.nlm.nih.gov/pubmed

• ScienceOpen
  ScienceOpen is a discovery platform with interactive features for scholars to improve their research, make an impact, and get credit. We help publishers build context for researchers.
  https://www.scienceopen.com/

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