Package natbib Error: Bibliography not compatible with author-year citations. ...mand\NAT@force@numbers{}\NAT@force@numbers

Question

I am using a Journal's format, however, receiving the following error:

Package natbib Error: Bibliography not compatible with author-year citations. ...mand\NAT@force@numbers{}\NAT@force@numbers.

How to get rid of this? The header of the LaTeX file is copied below for ready reference.

\documentclass[graybox, natbib, nosecnum]{svmult}
\bibpunct{(}{)}{;}{a}{}{,} % suppress commas between author-names and year
%\pdfoutput=1   %forces use of pdflatex. Disable if you prefer to use .eps 
or .ps figures.
% choose options for [] as required from the list
% in the Reference Guide

\usepackage{mathptmx}       % selects Times Roman as basic font
\usepackage{helvet}         % selects Helvetica as sans-serif font
\usepackage{courier}        % selects Courier as typewriter font
\usepackage{type1cm}        % activate if the above 3 fonts are
                            % not available on your system

\usepackage{makeidx}         % allows index generation
\usepackage{graphicx}        % standard LaTeX graphics tool
                         % when including figure files
\usepackage{multicol}        % used for the two-column index
\usepackage[bottom]{footmisc}% places footnotes at page bottom
\usepackage[normalem]{ulem} % for strike-through of text with \sout{}  
\usepackage{hyperref}  %for hyperlinks
\usepackage{soul}   % for high-lighting of text

\begin{document}
\begin{thebibliography}{99.}
% Book Chapter 
\bibitem{science-contrib} Broy, M.: Software engineering  from auxiliary to 
key technologies. In: Broy, M., Denert,
E. (eds.) Software Pioneers, pp. 10-13. Springer, New York (2002)
%
% Online Document
\bibitem{science-online} Cartwright, J.: Big stars have weather too. IOP 
Publishing PhysicsWeb.
\url{http://physicsweb.org/articles/news/11/6/16/1} (2007). Accessed 26 June 
2007
%
% Journal article
\bibitem{science-journal} Hamburger, C.: Quasimonotonicity, regularity and 
duality for nonlinear systems of partial
differential equations. Ann. Mat. Pura. Appl. 169, 321-354 (1995)
%
% Journal article by DOI
\bibitem{science-DOI} Slifka, M.K., Whitton, J.L.: Clinical implications of 
dysregulated cytokine production. J. Mol. Med. (2000) doi: 
10.1007/s001090000086 
%
% Monograph
\bibitem{science-mono} Geddes, K.O., Czapor, S.R., Labahn, G.: Algorithms 
for Computer Algebra. Kluwer, Boston (1992) 
%
\end{thebibliography}
\end{document}

Answer 1

The document class you employ automatically loads the natbib citation management package. There's also clearly a presumption that bibtex will be employed to create the bibliography. However, you're building the bibliography by hand. This implies, among other things, that you are in charge of every single aspect of what goes on inside the thebibliography environment.

In the process of building each formatted bibliographic item using BibTeX, natbib structures the header of each \bibitem in a very specific way, so that its \citet and \citep instructions may find the information they need to create correctly-formatted citation call-outs. If you build the thebibliography environment by hand, while still loading the natbib package, you must replicate this structure or -- as you've discovered -- be inundated by lots of warning and/or error messages, as well as be left with unusable citation call-outs.

Here are three suggestions for how you might proceed.

1. Don't load natbib, i.e., comment out (or delete) the option natbib and the \bibpunct instruction. Clearly, this is the lazy way. Note that this approach will generate numeric-style citation call-outs, which your journal may or may not accept.

2. Rewrite the headers of each \bibitem list item. E.g., instead of

   \bibitem{science-contrib} Broy, M.: ...
   ...
   \bibitem{science-DOI} Slifka, M.K., Whitton, J.L.: ...
   

   write

   \bibitem[Broy(2002)]{science-contrib} Broy, M.
   ...
   \bibitem[Slifka and Whitton(2000)]{science-DOI} Slifka, M.K., Whitton, J.L.: ...
   

   As I hope you have guessed by now, the material in square brackets is read by natbib in order to create authoryear-style citation call-outs to the entries with keys science-contrib and science-DOI.

3. Learn how to use BibTeX. In the medium to longer run, you'll save yourself a huge amount of time and effort. Concentrate on writing your papers, and delegate to BibTeX the task of creating properly and consistently-formatted bibliographies. You, and the readers of your papers, will benefit.